Renamed "media playing" APIs to "visible behind"

   - Request from API Review: rename the media playing APIs to a more
     generic name, reflecting the background visibility feature these
     methods actually control.
   - Made the new isActivityVisibleBehind().
   - Changed convertFromTranslucent() and convertToTranslucent() to be
     SystemApi.

Bug: 16959028
Change-Id: I526eac22f44273b3254dd6201f89194d13e597e2