SystemServer: Looper's getMainLooper to return ServerThread

Fix to ensure Context.getMainLooper and Looper.getMainLooper
returns ServerThread for system_server. Currently, Looper's
getMainLooper returns null.

Change-Id: I5324a1cc00e27a79ba97836a9377a584e6f76b15

Author: Ethan1983

services/java/com/android/server/SystemServer.java

@@ -79,7 +79,7 @@ public void run() {
         EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN,
             SystemClock.uptimeMillis());
 
-        Looper.prepare();
+        Looper.prepareMainLooper();
 
         android.os.Process.setThreadPriority(
                 android.os.Process.THREAD_PRIORITY_FOREGROUND);