Pipe volume keys to adjustVolume instead of sendMediaKeyEvent

We were calling sendMediaKeyEvent with a KEYCODE_VOLUME key, which was being
ignored because it's not a media key. This redirects the volume keys to use
the adjustVolume methods instead. It also sends the appropriate flags to
make the lock screen consistent with the home screen and the volume keys
only affect active playback when the screen is off.

bug:15900519
Change-Id: I9f3853a2385869353a58debae6e6ca9933ba06ce