fix ANRs for callbacks and when backgrounding app

Update to the OneSignal-Android-SDK 4.8.6 which includes both a fix for
ANRs the can happen when backgrounding the app and also provides a new
API to control which thread is used for fire callbacks.

By default the OneSignal-Android-SDK continues to fire callbacks on the
Main UI thread, however this SDK update introduces a new API
CallbackThreadManager to allow configuring these to a background thread

Author: jkasten2

com.onesignal.unity.android/Editor/OneSignalAndroidDependencies.xml

@@ -3,6 +3,6 @@
     <repositories>
       <repository>https://repo.maven.apache.org/maven2</repository>
     </repositories>
-    <androidPackage spec="com.onesignal:OneSignal:[4.8.5]" />
+    <androidPackage spec="com.onesignal:OneSignal:[4.8.6]" />
   </androidPackages>
 </dependencies>

com.onesignal.unity.android/Runtime/OneSignalAndroid.cs

@@ -111,6 +111,8 @@ public override void SetLaunchURLsInApp(bool launchInApp)
             => SDKDebug.Warn("This feature is only available for iOS.");
 
         public override void Initialize(string appId) {
+            SetOneSignalCallbacksToBackgroundThread();
+
             var unityPlayer = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
             var activity    = unityPlayer.GetStatic<AndroidJavaObject>("currentActivity");
 
@@ -137,6 +139,19 @@ public override void Initialize(string appId) {
             _completedInit(appId);
         }
 
+        // Change all OneSignal callbacks, observers, and events to fire on a background thread.
+        // This prevents cases where the main thread could hang waiting on Unity to bridge from Java back to C#.
+        private void SetOneSignalCallbacksToBackgroundThread() {
+            var callbackThreadManagerStr = SDKPackage + "." + "CallbackThreadManager";
+            var callbackThreadManagerClass = new AndroidJavaClass(callbackThreadManagerStr);
+            var callbackThreadManagerCompanionClass = callbackThreadManagerClass.GetStatic<AndroidJavaObject>("Companion");
+
+            var useThreadClass = new AndroidJavaClass(callbackThreadManagerStr + "$UseThread");
+            var useThreadBackgroundEnumValue = useThreadClass.GetStatic<AndroidJavaObject>("Background");
+
+            callbackThreadManagerCompanionClass.Call("setPreference", useThreadBackgroundEnumValue);
+        }
+
         public override async Task<NotificationPermission> PromptForPushNotificationsWithUserResponse() {
             var proxy = new PromptForPushNotificationPermissionResponseHandler();
             _sdkClass.CallStatic("promptForPushNotifications", true, proxy);