iOS: Make FlutterViewController.engine a strong ref

Previously, FlutterViewController.engine was declared as a weak readonly
property, but we explicitly declared the `FlutterEngine* _engine` ivar
as a strong reference in the implementation.

This changes the property declaration to be strong and eliminates the
now unnecessary ivar.

There is also no semantic change to FlutterViewController itself, since
the `_engine` ivar had been manually declared as a strong reference.

There is no semantic change for users of FlutterViewController.engine
since whether a user acquires a strong or weak reference to the engine
is determined by whether they declare the pointer to which they assign
it as strong or weak.

This also eliminates the need for the `engine` getter, which was only
present to prevent a warning that the strong ivar didn't match the weak
property declaration.

No changes to tests since this introduces no semantic changes.

Issue: flutter/flutter#137801

Author: cbracken

shell/platform/darwin/ios/framework/Headers/FlutterViewController.h

@@ -233,7 +233,7 @@ FLUTTER_DARWIN_EXPORT
  * `FlutterViewController` is initialized with or a new `FlutterEngine` implicitly created if
  * no engine was supplied during initialization.
  */
-@property(weak, nonatomic, readonly) FlutterEngine* engine;
+@property(nonatomic, readonly) FlutterEngine* engine;
 
 /**
  * The `FlutterBinaryMessenger` associated with this FlutterViewController (used for communicating

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -153,7 +153,6 @@ - (void)handleKeyboardAnimationCallbackWithTargetTime:(fml::TimePoint)targetTime
 @end
 
 @implementation FlutterViewController {
-  FlutterEngine* _engine;
   flutter::ViewportMetrics _viewportMetrics;
   MouseState _mouseState;
 }
@@ -302,10 +301,6 @@ - (void)performCommonViewControllerInitialization {
   [self setUpNotificationCenterObservers];
 }
 
-- (FlutterEngine*)engine {
-  return _engine;
-}
-
 - (void)setUpNotificationCenterObservers {
   NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
   [center addObserver:self