Bug 837236 - share video decoding hardware among Camera, Gallery and Video r=daleharvey

part 1: make camera save a poster image for videos

part 2: remove videodb from gallery app

part 3: update Gallery metadata parser to use video poster images

part 4: modify media_frame and video_player to use a poster image

part 5: update camera to use new media_frame api

part 6: modify Video to release video hardware when it changes visibility

part 7: defer Video metadata parsing so Camera has time to create poster image

part 8: give Gallery permission to use the hardware video decoder

part 9: modify Video to delete poster image when deleting video