setActiveTrack and setBufferProvider can't fail

Return void, not status_t, from setActiveTrack and setBufferProvider.

These methods returned status_t, but the callers never checked the
return value.  Since these aren't externally visible APIs, they now
return void, and assert on bad input parameters.

Change-Id: I530ed29484596ae41e8659826ca425149c51c2a1