Adds a mechanism to listen to proxy transact method calls.

There are multiple use cases for it:

1) Make it easy for another process to set the worksource. The
worksource can be propagated in a thread local - this is how gmscore and soon
system server works -  the worksource can then be set for all binder
calls using

Object transactStarted() {
    Binder.setWorkSource(ThreadLocalWorkSourceUid.get());
    return null;  // No token needed.
}

void transactEnded() {
    Binder.setWorkSource(null);
}

This will be used by system process and gmscore.

2) SystemUI team was interested in detecting binder calls done from the
main thread in dogfood/tests. This listener will make it easy to figure
out which thread is used.

Performance impact of transact method:
    - With current code: 45ns per call
    - With this code: 57ns per call
This is not significant compared to the total binder call time which is
10-100s of microseconds.

Test: unit test
Change-Id: Id0a2f52cba33b390ff83f703284b79471cc80b1c