[Mono.Android] ResourceIdManager.UpdateIdValues called N times (#1855)

Fixes: #1544

I was able to verify that my app's `Resource.UpdateIdValues()` was
getting invoked multiple times with VS 15.7.4:

  - Create a new Xamarin.Android app
  - Create a new Xamarin.Android library, reference it from the app
  - (Optional) call
    `Android.Runtime.ResourceIdManager.UpdateIdValues()` directly

By placing a breakpoint, and/or `Log.Debug()` messages, I can verify
that my app's `Resource.UpdateIdValues()` was being called 5 times!

The issue lies in this code:

    if (id_initialized)
        return;
    //System.Reflection code to call Resource.UpdateIdValues() in the app
    id_initialized = true;

While `Resource.UpdateIdValues()` is being invoked,
`ResourceIdManager.UpdateIdValues()` is called recursively.  The
`id_initialized` field will not be set until after the reflection
code has finished.

The fix is to immediately set the flag:

    if (id_initialized)
        return;
    id_initialized = true;
    //System.Reflection code to call Resource.UpdateIdValues() in the app

I was able to verify the fix in my app:

  - Build the app with `xabuild`
  - Re-add the `Log.Debug` message in my app's
    `Resource.UpdateIdValues()`
  - Deploy the app with `xabuild`
  - Upon launch, I *now* only see 1 log message with `adb logcat`

The performance impact is likely:

  - The reflection code would be slow in
    `Android.Runtime.ResourceIdManager.UpdateIdValues()`
  - In the app's `Resource.UpdateIdValues()`, mostly fields are being
    read and set. It *helps* to skip this, but it shouldn't be as
    slow as the reflection code.

Author: jonathanpeppers

src/Mono.Android/Android.Runtime/ResourceIdManager.cs

@@ -10,6 +10,7 @@ public static void UpdateIdValues ()
 		{
 			if (id_initialized)
 				return;
+			id_initialized = true;
 			var executingAssembly = Assembly.GetExecutingAssembly ();
 			var type = executingAssembly != null ? GetResourceTypeFromAssembly (executingAssembly) : null;
 			if (type == null) {
@@ -27,7 +28,6 @@ public static void UpdateIdValues ()
 					action ();
 				}
 			}
-			id_initialized = true;
 		}
 
 		static Type GetResourceTypeFromAssembly (Assembly assembly)