Do alert work in background thread

* An ANR was reported in MailboxAlarmReceiver
* The reciever calls into SyncManager, which does some database
  operations, and may abort an I/O operation in a sync service
  thread
* Move this potentially long-running code into a background
  thread

Bug: 2215045
Change-Id: Id65c51f706b212d6b50af3921f3ba3dc2d014ce0
1 file changed