Change InCallTonePlayerTest to use mock MediaPlayer.

This was causing flakes.  Using a mock will prevent that.
Unfortunately the startPlaying method on MediaPlayer is native, so had to
abstract out access to MediaPlayer.

Bug: 120289096
Bug: 120422950
Test: Ran unit tests.
Merged-In: If5462ae23978238e948f8693c95813b0dc4a1850
Change-Id: If5462ae23978238e948f8693c95813b0dc4a1850