audio policy: use client descriptors

First step in implementing use of ClientDescriptor class to keep track
of playback and capture activity:
- update AudioPolicyInterface methods to just use port ID as
client identifier
- add list of clients in input and output descriptors and update list
when clients are added and removed.
- list clients in audio policy manager dump.

Test: Audio smoke tests. CTS tests for AudioTrack, AudioRecord, routing, recording and playback configurations

Change-Id: Ia0f62f295d421fb845d57afcaa0ce77cd2c58775