Camera: Cleanup hardware key shutter handling

* Fix power shutter behaviour in camcorder mode
* Disable volume change while in camcorder mode, volume buttons should only
handle camera operations while in the camera

On devices with a hardware camera key:
* Disable power & volume shutter features
* Volume buttons control the zoom

On devices without a hardware camera key:
* When power shutter is disabled, volume buttons control the shutter (like AOSP)
* When power shutter is enabled, volume buttons control the zoom

Change-Id: I141946d5b6f5aec8b1e9a9f99270d30295596079