audio policy: allows dumping Audio Policy Mix

This patch adds the dump of AudioPolicyMix if registered.
It also adds the required conversion map to human readable strings
of the types involved in Mixes.

Adapted for the development branch from the AOSP patch:
https://android-review.googlesource.com/#/c/309127/

Change-Id: I91cf7afe6de5ff837970d9063ca437d4f6201e89
Test: build, dumpsys media.audio_policy