Notification playback synchronized with audio focus

PlayerProxy: more control options (pan, delayed start),
  remove exceptions in method signatures.
Use delayed start on notification playback for better sync
  between media apps and notifications
Disabled for now: support for AudioService (through
  MediaFocusControl and PlaybackActivityMonitor) to enforce
  audio ducking of focus owners losing audio focus with
  AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK, and who don't
  play audio that has a content labelled as CONTENT_TYPE_SPEECH.
  This feature will be enabled when setting a VolumeShaper
  on a player works.

Test: play music and play notification
Bug 30258418

Change-Id: I4e4d911645306bbde17f74288f3b61781fe0e3fe