Expose capture state listener in AudioSystem

This is a client-side wrapper around the respective feature of APS.

Bug: 146157104
Test: Manual verification of proper behavior when starting/stopping
      recording, killing of audio server, by examination of the logs.
Merged-In: I9b56c431f69e77683151de19830ec6f45741dd6f
Change-Id: I1f5279c4a4b18e0a7862d3b103d383ac3de4c9a0