audiopolicy: refactor playback activity ref counting

Playback activity ref couting is now based on active state
of PlaybackClientDescriptor.

The active reference count per stream on each output is still
used because it provides a simple implementation for duplicated output.

Test: CTS tests for AudioTrack and Routing
Test: manual audio smoke test

Change-Id: Ifadb1d24131581461aace14a5b25e7c7582c57cb