Audio event logging

New class AudioEventLogger to store a configurable number of the
  last audio events. It hosts a circular buffer of lightweight
  event objects, an abstract class whose implementations are
  event-specific, and does the heavier string manipulations
  at the time of the dump, not during normal operation.
Add logging for:
  - phone state (a.k.a. audio mode) changes
  - wired device connections
  - playback activity monitor
  - force use

Test: adb shell dumpsys audio
Bug: 64470715
Change-Id: I8dbd936e2ffec39134f1a8837191c74f5cb9fc56