Refactor playback position tracking

Use PlaybackState directly to track the position through time.  Update
the state when necessary (track changes, etc.)

Remove complex logic and centralize sending of Play Position
Notificaiton response and scheduling.

Update dumpsys and add current package session.

Bug: 29416450
Bug: 29606822

Change-Id: Ie82b48d6d7f9e1a05b64835731433d05a3a3a41a
(cherry picked from commit eb9e6ef0)