Rewrite of MediaPlayer logic.

+ Control MediaPlayer instance more tightly. Wait until
prepareContent to initialize instance. Release MediaPlayer when it
is no longer needed.
+ Instead of using isFinishing, check explicitly for orientation
change to know whether to release MediaPlayer.
+ Change Presenter to singleton, to address audio change wonkiness.
+ Only create a Presenter if the call log fragment shows voicemail.
+ ... fixing a variety of cases.

- Temporarily disable proximity sensor until blocking issue is fixed.

Bug: 21856243
Change-Id: Ic06e98bb5278467c3cce726a06b6cf3d855861a2