[Mono.Android] Dispose of the RunnableImplementor on error (#8907)

Context: 3ce27c9

Commit 3ce27c9 had a TODO:

> TODO: Address [^0] and dispose of the `RunnableImplementor` instance
> when `View.Post()` returns `false`.
>
> [^0]: we leak if `View.post(Runnable)` returns *false*.

The time is now!  Review all `new RunnableImplementor(…, true)` calls,
and if a "removable" `RunnableImplementor` is passed to a method
which can return an error such as [`Handler.post()`][0] or
[`View.post()`][1], dispose of the `RunnableImplementor` instance
on error.

[0]: https://developer.android.com/reference/android/os/Handler#post(java.lang.Runnable)
[1]: https://developer.android.com/reference/android/view/View#post(java.lang.Runnable)

src/Mono.Android/Android.OS/Handler.cs

@@ -13,27 +13,52 @@ public Handler (Action<Message> handler)
 
 		public bool Post (Action action)
 		{
-			return Post (new Java.Lang.Thread.RunnableImplementor (action, true));
+			var runnable    = new Java.Lang.Thread.RunnableImplementor (action, removable: true);
+			if (Post (runnable)) {
+				return true;
+			}
+			runnable.Dispose ();
+			return false;
 		}
 
 		public bool PostAtFrontOfQueue (Action action)
 		{
-			return PostAtFrontOfQueue (new Java.Lang.Thread.RunnableImplementor (action, true));
+			var runnable    = new Java.Lang.Thread.RunnableImplementor (action, removable: true);
+			if (PostAtFrontOfQueue (runnable)) {
+				return true;
+			}
+			runnable.Dispose ();
+			return false;
 		}
 
 		public bool PostAtTime (Action action, long uptimeMillis)
 		{
-			return PostAtTime (new Java.Lang.Thread.RunnableImplementor (action, true), uptimeMillis);
+			var runnable    = new Java.Lang.Thread.RunnableImplementor (action, removable: true);
+			if (PostAtTime (runnable, uptimeMillis)) {
+				return true;
+			}
+			runnable.Dispose ();
+			return false;
 		}
 
 		public bool PostAtTime (Action action, Java.Lang.Object token, long uptimeMillis)
 		{
-			return PostAtTime (new Java.Lang.Thread.RunnableImplementor (action, true), token, uptimeMillis);
+			var runnable    = new Java.Lang.Thread.RunnableImplementor (action, removable: true);
+			if (PostAtTime (runnable, token, uptimeMillis)) {
+				return true;
+			}
+			runnable.Dispose ();
+			return false;
 		}
 
 		public bool PostDelayed (Action action, long delayMillis)
 		{
-			return PostDelayed (new Java.Lang.Thread.RunnableImplementor (action, true), delayMillis);
+			var runnable    = new Java.Lang.Thread.RunnableImplementor (action, removable: true);
+			if (PostDelayed (runnable, delayMillis)) {
+				return true;
+			}
+			runnable.Dispose ();
+			return false;
 		}
 
 		public void RemoveCallbacks (Action action)

src/Mono.Android/Android.Views/View.cs

@@ -49,12 +49,22 @@ public T RequireViewById<
 
 		public bool Post (Action action)
 		{
-			return Post (new Java.Lang.Thread.RunnableImplementor (action, true));
+			var runnable    = new Java.Lang.Thread.RunnableImplementor (action, removable: true);
+			if (Post (runnable)) {
+				return true;
+			}
+			runnable.Dispose ();
+			return false;
 		}
 
 		public bool PostDelayed (Action action, long delayMillis)
 		{
-			return PostDelayed (new Java.Lang.Thread.RunnableImplementor (action, true), delayMillis);
+			var runnable    = new Java.Lang.Thread.RunnableImplementor (action, removable: true);
+			if (PostDelayed (runnable, delayMillis)) {
+				return true;
+			}
+			runnable.Dispose ();
+			return false;
 		}
 
 		public bool RemoveCallbacks (Action action)