[generator] Remove androidx.annotation from Annotation names

[jonpryor]

Context: dotnet/android#6300

A confluence of several "funny" things happened with
dotnet/android#6300, which attempts to use the Android SDK
platform-tools 31.0.3 package, up from 30.0.2:

1. The new platform-tools package removes the file
   platform-tools/api/annotations.zip. To work around that, we
   need to instead use the data/annotations.zip file from the
   Android platform directory, e.g.
   $ANDROID_SDK_ROOT/platforms/android-30/data/annotations.zip.

2. Between API-28 and API-29, annotations.zip changes the package
   name used for annotations, from e.g.
   android.support.annotation.RequiresPermission to
   androidx.annotation.RequiresPermission.

In isolation, okay, but then we hit:

3. AnnotationData removes the "known" package-prefix of
   android.support.annotation., and all use of
   AnnotationData.Name is via string.operator==, not
   string.Contains() or string.IndexOf().

The result of all these changes together is API breakage in
Mono.Android.dll between API-28 and API-29:

…/Mono.Android.targets(257,5): error :
CannotRemoveAttribute : Attribute 'Android.Runtime.RequiresPermissionAttribute' exists on
'Android.Accounts.AccountManager.AddAccount(System.String, System.String, System.String[], Android.OS.Bundle, Android.App.Activity, Android.Accounts.IAccountManagerCallback, Android.OS.Handler)'
in the contract but not the implementation.

Or, in C# diff -u terms:

--- obj/Debug/monoandroid10/android-28/mcw/Android.Accounts.AccountManager.cs	2021-09-16 09:00:53.000000000 -0400
+++ obj/Debug/monoandroid10/android-29/mcw/Android.Accounts.AccountManager.cs	2021-09-16 10:41:15.000000000 -0400
@@ -217,7 +217,6 @@

 		// Metadata.xml XPath method reference: path="/api/package[@name='android.accounts']/class[@name='AccountManager']/method[@name='addAccount' and count(parameter)=7 and parameter[1][@type='java.lang.String'] and parameter[2][@type='java.lang.String'] and parameter[3][@type='java.lang.String[]'] and parameter[4][@type='android.os.Bundle'] and parameter[5][@type='android.app.Activity'] and parameter[6][@type='android.accounts.AccountManagerCallback<android.os.Bundle>'] and parameter[7][@type='android.os.Handler']]"
 		[Register ("addAccount", "(Ljava/lang/String;Ljava/lang/String;[Ljava/lang/String;Landroid/os/Bundle;Landroid/app/Activity;Landroid/accounts/AccountManagerCallback;Landroid/os/Handler;)Landroid/accounts/AccountManagerFuture;", "GetAddAccount_Ljava_lang_String_Ljava_lang_String_arrayLjava_lang_String_Landroid_os_Bundle_Landroid_app_Activity_Landroid_accounts_AccountManagerCallback_Landroid_os_Handler_Handler")]
-		[global::Android.Runtime.RequiresPermission ("android.permission.MANAGE_ACCOUNTS")]
 		public virtual unsafe Android.Accounts.IAccountManagerFuture? AddAccount (string? accountType, string? authTokenType, string[]? requiredFeatures, Android.OS.Bundle? addAccountOptions, Android.App.Activity? activity, Android.Accounts.IAccountManagerCallback? @callback, Android.OS.Handler? handler)
 		{
 			const string __id = "addAccount.(Ljava/lang/String;Ljava/lang/String;[Ljava/lang/String;Landroid/os/Bundle;Landroid/app/Activity;Landroid/accounts/AccountManagerCallback;Landroid/os/Handler;)Landroid/accounts/AccountManagerFuture;";

Fix this breakage by updating AnnotationData to remove the package
name for androidx.annotation as well.

Additionally, add an AnnotatedItem.ToString() override to simplify
future printf-style debugging.