Stride.Games/Android: guard ringer receiver register/unregister against pause-before-OnRun

Launching while the device is asleep fires onResume/onPause on the UI thread
before Silk's OnRun (separate thread) created the receiver, so OnPause called
UnregisterReceiver with a null field and crashed. Create the receiver lazily and
track registration under a lock so the two threads stay balanced.

Author: xen2

sources/engine/Stride.Games/Starter/StrideActivity.cs

@@ -57,10 +57,6 @@ protected override void OnDestroy()
 
         protected override void OnRun()
         {
-            // set up a listener to the android ringer mode (Normal/Silent/Vibrate)
-            ringerModeIntentReceiver = new RingerModeIntentReceiver((AudioManager)GetSystemService(AudioService));
-            RegisterReceivers();
-
             // Set the android global context
             if (PlatformAndroid.Context == null)
                 PlatformAndroid.Context = this;
@@ -170,8 +166,11 @@ internal void HideEditTextPopup(PopupWindow popupWindow)
             });
         }
 
+        // Created lazily here (called from OnResume), not in OnRun: OnRun is on a separate thread
+        // that may not have run when a launch-while-asleep pauses us, which would unregister a null.
         private void RegisterReceivers()
         {
+            ringerModeIntentReceiver ??= new RingerModeIntentReceiver((AudioManager)GetSystemService(AudioService));
             var ringerModeIntentFilter = new IntentFilter(AudioManager.RingerModeChangedAction);
             if (OperatingSystem.IsAndroidVersionAtLeast(34))
             {