Settings: set volume wake dependency in code

It is possible for volume_wake_screen to be removed from the preference
list, but volbtn_music_controls has a hard dependency on
volume_wake_screen. Let's setup this depenency only if both exist.

Change-Id: I18b2ecc0de36779ebdea3a0d20c1971ce85549cb
Signed-off-by: Roman Birg <roman@cyngn.com>