Prevent wakelock from being held when sync services finish

* Make sure that ExchangeService#done is called when all threads
  exit
* Make ExchangeService#done ensure that the calling thread is
  still the active syncing thread for its mailbox before taking
  action
* For MR0/MR1

Bug: 5256268
Change-Id: I64bc5f2539c524f3b8d633117ca1c548a373e449
2 files changed