Simplify TransactionCallbackInvoker

The old code was written around the old threading model, and had
to deal with callback registration coming from multiple binder
threads that could interleave with sendCallbacks. The old code
likewise had to deal with cases where a Transaction would apply
but not "latch" in this frame. Both of these cases are removed
by queued transactions, and so we can rework the model to be much
simpler. This is the first step towards another simplification
used to rework the frame queuing model.

Test: Existing tests pass
Bug: 201436596
Change-Id: I6983221ce66bb31c5da1122bde176ab4465556eb