Slightly shorten time before we show a foreground notification for pushes.

There's some ANRs, not many, that are likely caused by us riding the
5-second ANR timeout a little too closely. Giving us a little buffer to
see if that helps.

src/org/thoughtcrime/securesms/gcm/GcmBroadcastReceiver.java

@@ -121,7 +121,7 @@ public class GcmBroadcastReceiver extends WakefulBroadcastReceiver implements In
       new Thread("GcmForegroundServiceTimer") {
         @Override
         public void run() {
-          Util.sleep(5000);
+          Util.sleep(4500);
           synchronized (foregroundLock) {
             if (!taskCompleted.get() && !foregroundRunning.getAndSet(true)) {
               Log.i(TAG, "Starting a foreground task because the job is running long.");