Release security holds based on flags; implement remote wipe

* When the UI indicates that security policies for a particular
  account are now in force and releases the security hold (a bit
  in the Account flags), we release any syncs that had been
  waiting due to security errors
* Clean up code related to sync holds
* Add unit test for sync hold release
* Add support for server-directed remote wipe

Change-Id: I6209f75e4b57c850ae1ac27f407630c9c740514f
4 files changed