add public CallbackThreadManager class

This new public API allows changing the default thread used to fire aync
events, observers, and callbacks for the rest of the OneSignal public
API.

This is intended for advanced use-case, motivated
mostly a need of the OneSignal Unity wrapper SDK. Specifically to
avoid cases where Unity may cause the main UI thread to wait on a
background thread when calling back into Unity. Given how the binding
between C# and Java works here, there is no way for the C# code to
switch threads itself, so this API has to be available to do so.

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/CallbackThreadManager.kt

@@ -0,0 +1,59 @@
+/**
+ * Modified MIT License
+ * <p>
+ * Copyright 2023 OneSignal
+ * <p>
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ * <p>
+ * 1. The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ * <p>
+ * 2. All copies of substantial portions of the Software may only be used in connection
+ * with services provided by OneSignal.
+ * <p>
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.onesignal
+
+import kotlin.concurrent.thread
+
+/**
+ * Provides a public API to allow changing which thread callbacks and observers
+ * should fire on.
+ *
+ * Initial motivation for this is to allow the OneSignal-Unity-SDK to config
+ * the SDK to fire off the main thread. This is to avoid cases where Unity may
+ * cause the main UI thread to wait on a background thread when calling back
+ * into Unity.
+ *
+ * Usage: CallbackThreadManager.preference = UseThread.Background
+ */
+class CallbackThreadManager {
+    enum class UseThread {
+        MainUI,
+        Background
+    }
+
+    companion object {
+        var preference = UseThread.MainUI
+
+        fun runOnPreferred(runnable: Runnable) {
+            when (preference) {
+                UseThread.MainUI -> OSUtils.runOnMainUIThread(runnable)
+                UseThread.Background -> thread { runnable.run() }
+            }
+        }
+    }
+}

OneSignalSDK/onesignal/src/main/java/com/onesignal/OSInAppMessageController.java

@@ -479,7 +479,7 @@ private void firePublicClickHandler(@NonNull final String messageId, @NonNull fi
         if (OneSignal.inAppMessageClickHandler == null)
             return;
 
-        OSUtils.runOnMainUIThread(new Runnable() {
+        CallbackThreadManager.Companion.runOnPreferred(new Runnable() {
             @Override
             public void run() {
                 // Send public outcome from handler

OneSignalSDK/onesignal/src/main/java/com/onesignal/OSObservable.java

@@ -79,7 +79,7 @@ boolean notifyChange(final StateType state) {
                final Method method = clazz.getDeclaredMethod(methodName, state.getClass());
                method.setAccessible(true);
                if (fireOnMainThread) {
-                  OSUtils.runOnMainUIThread(
+                  CallbackThreadManager.Companion.runOnPreferred(
                       new Runnable() {
                           @Override
                           public void run() {

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignal.java

@@ -2332,7 +2332,7 @@ private static void fireNotificationOpenedHandler(final OSNotificationOpenedResu
 
       // TODO: Once the NotificationOpenedHandler gets a Worker, we should make sure we add a catch
       //    like we have implemented for the OSRemoteNotificationReceivedHandler and NotificationWillShowInForegroundHandlers
-      OSUtils.runOnMainUIThread(new Runnable() {
+      CallbackThreadManager.Companion.runOnPreferred(new Runnable() {
          @Override
          public void run() {
             notificationOpenedHandler.notificationOpened(openedResult);

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalStateSynchronizer.java

@@ -306,8 +306,7 @@ public void onComplete(String channel, boolean success) {
                }
             }
 
-            // Need to call completion handler on main thread since the request response came from an async PUT
-            OSUtils.runOnMainUIThread(new Runnable() {
+            CallbackThreadManager.Companion.runOnPreferred(new Runnable() {
                @Override
                public void run() {
                   if (completionHandler != null)