Fix remote wipe with mobile sync server

* It appears as if our running multiple sync threads can confuse the
  mobile sync server during a remote wipe (the server expects the next
  client response to be an acknowledgment, whereas it might well be
  a command or response from a different thread)
* To avoid this, we first put the account on security hold and then
  shut down all other sync threads for the account
* After this, we send the acknowledgment and the remote wipe proceeds
  normally.
* NOTE: It's possible that, due to the vagaries of multithreaded
  operation, one of the other syncing threads could still send a non-
  acknowledgment response to the server before our provisioning thread
  gets a chance to send its acknowledgment.  However, since the other
  syncing threads will terminate (and not restart, because of the hold),
  the provision/remote wipe/ack sequence will work on the subsequent
  attempt

Bug: 2844888
Backport From: Ib4ffbbc67b681e69176b6c1d5515fa80c7d1e121

Change-Id: Ie9e944bd39f331c2ddc0f0ba303a3d5684f6f033
3 files changed