Fix deadlock.

AccountManagerService can't ever synchronize on mUsers within a block of
code locked by UserAccounts.cacheLock. That will lead to deadlocks.

This change fixes a case where we were doing that in
getAccountsInternal(). Also I have purgeOldGrantsAll() run off the the
main thread.

Bug: 23036400
Change-Id: I8634691ca54c57a6e83633baba549226fdcd1064