Fix unguarded availability warnings in Messaging

[paulb777]

Split from #385

Xcode 9 introduces warnings for unguarded API calls introduced in later versions.

Occurrences show up in Auth and Messaging:

$ pod spec lint FirebaseCommunity.podspec

-> FirebaseCommunity (0.1.4)
- WARN | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthAppDelegateProxy.m:122:10: warning: 'UIApplicationOpenURLOptionsAnnotationKey' is only available on iOS 9.0 or newer [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Auth] xcodebuild: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator11.0.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIApplication.h:535:51: note: 'UIApplicationOpenURLOptionsAnnotationKey' has been explicitly marked partial here
- NOTE | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthAppDelegateProxy.m:122:10: note: enclose 'UIApplicationOpenURLOptionsAnnotationKey' in an @available check to silence this warning
- WARN | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:47:3: warning: 'SFSafariViewController' is partial: introduced in iOS 9.0 [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Auth] xcodebuild: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator11.0.sdk/System/Library/Frameworks/SafariServices.framework/Headers/SFSafariViewController.h:27:12: note: 'SFSafariViewController' has been explicitly marked partial here
- NOTE | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.h:41:12: note: annotate 'FIRAuthURLPresenter' with an availability attribute to silence
- WARN | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:104:40: warning: 'SFSafariViewController' is partial: introduced in iOS 9.0 [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:104:1: note: annotate 'safariViewControllerDidFinish:' with an availability attribute to silence
- WARN | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:80:10: warning: 'SFSafariViewController' is only available on iOS 9.0 or newer [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:80:10: note: enclose 'SFSafariViewController' in an @available check to silence this warning
- WARN | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:81:33: warning: 'SFSafariViewController' is only available on iOS 9.0 or newer [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:81:33: note: enclose 'SFSafariViewController' in an @available check to silence this warning
- WARN | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:164:3: warning: 'SFSafariViewController' is only available on iOS 9.0 or newer [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Auth] xcodebuild: FirebaseCommunity/Firebase/Auth/Source/FIRAuthURLPresenter.m:164:3: note: enclose 'SFSafariViewController' in an @available check to silence this warning
- WARN | [FirebaseCommunity/Database] xcodebuild: leveldb-library/db/c.cc:274:56: warning: possible misuse of comma operator here [-Wcomma]
- NOTE | [FirebaseCommunity/Database] xcodebuild: leveldb-library/db/c.cc:274:21: note: cast expression to void to silence warning
- WARN | [FirebaseCommunity/Database] xcodebuild: leveldb-library/db/c.cc:275:56: warning: possible misuse of comma operator here [-Wcomma]
- NOTE | [FirebaseCommunity/Database] xcodebuild: leveldb-library/db/c.cc:275:21: note: cast expression to void to silence warning
- WARN | [FirebaseCommunity/Database] xcodebuild: leveldb-library/db/version_set.cc:1350:5: warning: code will never be executed [-Wunreachable-code]
- NOTE | [FirebaseCommunity/Database] xcodebuild: leveldb-library/db/version_set.cc:1349:7: note: silence by adding parentheses to mark code as explicitly dead
- WARN | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/FIRMessaging.m:407:18: warning: 'application:openURL:options:' is only available on iOS 9.0 or newer [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Messaging] xcodebuild: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator11.0.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIApplication.h:326:1: note: 'application:openURL:options:' has been explicitly marked partial here
- NOTE | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/FIRMessaging.m:407:18: note: enclose 'application:openURL:options:' in an @available check to silence this warning
- WARN | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/FIRMessaging.m:699:20: warning: 'messaging:didReceiveMessage:' is only available on iOS 10.0 or newer [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/Public/FIRMessaging.h:260:1: note: 'messaging:didReceiveMessage:' has been explicitly marked partial here
- NOTE | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/FIRMessaging.m:699:20: note: enclose 'messaging:didReceiveMessage:' in an @available check to silence this warning
- WARN | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/FIRMessagingContextManagerService.m:147:20: warning: 'setAlertTitle:' is only available on iOS 8.2 or newer [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Messaging] xcodebuild: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator11.0.sdk/System/Library/Frameworks/UIKit.framework/Headers/UILocalNotification.h:44:47: note: 'setAlertTitle:' has been explicitly marked partial here
- NOTE | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/FIRMessagingContextManagerService.m:147:20: note: enclose 'setAlertTitle:' in an @available check to silence this warning
- WARN | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/FIRMessagingRmq2PersistentStore.m:107:26: warning: 'sqlite3_errstr' is only available on iOS 8.2 or newer [-Wunguarded-availability]
- NOTE | [FirebaseCommunity/Messaging] xcodebuild: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator11.0.sdk/usr/include/sqlite3.h:3385:24: note: 'sqlite3_errstr' has been explicitly marked partial here
- NOTE | [FirebaseCommunity/Messaging] xcodebuild: FirebaseCommunity/Firebase/Messaging/FIRMessagingRmq2PersistentStore.m:107:26: note: enclose 'sqlite3_errstr' in an @available check to silence this warning

Analyzed 1 podspec.

[ryanwilson]

Had to roll the Messaging change back for the 4.8.0 release due to @available only being available in Xcode 9.

TODO: use +#if defined(__IPHONE_11_0) && (__IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_11_0) or something like that to check if the @available function is available. Otherwise, break out into helper functions.