Synchronize access to mCallbacksList

If a transaction timeout occurs while another callback is being invoked,
it's possible that a race condition will occur where beginBroadcast will
be invoked twice which will cause a crash.

Fixes: 229657469
Test: PTS
Change-Id: I713c3f978a98f3db1d109233bb4ee1c0ab60557c
1 file changed