Synchronize broadcasts so that beginBroadcast does not get called twice

Synchronize the callback broadcasts to ensure that beginBroadcast
and endBroadcast is called before the next beginBroadcast is called.

Bug: 276374973
Test: manual calls/sms, presubmit
Change-Id: I3e247e1b3b123e31050969bca01296352cc6ea69