[gbinder] Fixed a threading issue. JB#42956

Make sure that transaction handlers don't get invoked on a worker thread.
It was possible in cases if an incoming transaction arrives while we are
waiting for response from an asynchronous call to servicemanager (which
is actually a synchronous call made on a worker thread).

Invoking callbacks on a worker thread would completely break libgbinder's
synchronization model. It was a pretty unlikely scenario, though.