Add locking around SyncManager accounts handling.

The running accounts managed within SyncManager were not protected
behind a lock and caused race conditions when adding new accounts.
This CL adds a lock around the accounts array to avoid such race
conditions.

Bug: 165011202
Test: manual (add new account, observe no "account doesn't exist" errors)
Test: SyncManagerTest
Test: CtsSyncManagerTest
Change-Id: I8ce2a6f93a913bb3cd55ca7a1e292414d45834f6