TransactionCompletedListener: Mutex for getInstance

We have always needed this technically, since otherwise
calling getInstance from multiple threads could fail. It seems
before we were lucky enough to not actually do that, until
the situation in the attached bug arose.

Bug: 226882766
Test: Existing tests pass
Change-Id: I648f6fa48296599a0c0e0d00a07fb71d9330880d
1 file changed