Move BackgroundRunnable to it's own file

Author: Jeasmine

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

@@ -0,0 +1,9 @@
+package com.onesignal
+
+import android.os.Process
+
+internal open class BackgroundRunnable : Runnable {
+    override fun run() {
+        Thread.currentThread().priority = Process.THREAD_PRIORITY_BACKGROUND
+    }
+}

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

@@ -251,13 +251,6 @@ private void runRunnableOnThread(Runnable runnable) {
             runnable.run();
     }
 
-    private static class BackgroundRunnable implements Runnable {
-        @Override
-        public void run() {
-            Thread.currentThread().setPriority(Process.THREAD_PRIORITY_BACKGROUND);
-        }
-    }
-
     interface InvalidOrDuplicateNotificationCallback {
 
         /**