Make SyncManager and AccountManagerService multi-user aware.

AccountManagerService
- Maintain multiple account lists, one per user
- Keep multiple databases of accounts
- Account db moved to /data/system/users/<userid>/

SyncManager
- SyncStorageEngine keeps track of multiple users' accounts.
- SyncQueue maintained as a single instance, queueing requests from
  multiple users.
- Changed some methods to take userId arguments
- Removed some deadc0de
- Store the userId in the SyncOperation, so we know which provider
  instance to bind to when queued operations are processed.

ContentService
- Pass along the userid to sync manager calls.

ActivityManagerService:
- Fixed a bug in cancelIntentSender
- Don't bring other user's task forward when resetting tasks.

Updated tests

Change-Id: If317340ef68e902787aa3f5ceb4cf96f14aea695