Show Camera placeholder as first filmstrip item in roll

Bug: 7272674
The camera roll filmstrip view now shows a shortcut to
launch the camera when opened from the Gallery app.
Note: the edits to SnailAlbum and SnailSource were made to
allow sharing code between SingleItemAlbum and SnailAlbum

Change-Id: I1bd2f3db99138c4b79a41c9c5baee46704cd18e0