ActivityThread: Avoid creating duplicate handlers and reuse mH

sMainThreadHandler is used for notification of changes in
SharedPreferences and refers to a new instance of Handler instead
of the existing instance, mH, of the ActivityThread. Fix to reuse
existing handlers associated with main looper.

Change-Id: I794a5802a9eacdb188bd0619db5e70e3ae89a07d

Author: Ethan1983

core/java/android/app/ActivityThread.java

@@ -4728,13 +4728,14 @@ public static void main(String[] args) {
         Process.setArgV0("<pre-initialized>");
 
         Looper.prepareMainLooper();
-        if (sMainThreadHandler == null) {
-            sMainThreadHandler = new Handler();
-        }
 
         ActivityThread thread = new ActivityThread();
         thread.attach(false);
 
+        if (sMainThreadHandler == null) {
+            sMainThreadHandler = thread.getHandler();
+        }
+
         AsyncTask.init();
 
         if (false) {