Fix deadlock by making DropBoxManager call async.

Created a Handler and used it to make sendBroadcast call asynchronously.
Deadlock was caused by WindowManagerService Log.wtf call requiring
ActivityManagerService lock while holding its own lock. At the same time
ActivityManagerService was holding its lock while waiting for
WindowManagerService lock.

Tested by forcing a Log.wtf in
WindowManagerServices.updateWindowsAppsAndRotationAnimationsLocked
inside mAppTokens loop. Then ran 'adb shell monkey -v -v 500000'.
Without this fix it would lock up and reboot within a couple of minutes.
With this fix it runs until the Camera app crashes. But that's a
different bug...

Fixes bug 6112676.

Change-Id: I5b360aa08412d117b1765f01bacd931020509db7