Add opt-out mechanism for wallpaper scrolling.

This allows disabling it on devices that are too slow for it.
Also add some comments for the config values and make the name for the
recolor enable flag default more clear.

Change-Id: Id853697de388daea48c0b4387e51cc4127c37e04
Merge "Add opt-out mechanism for wallpaper scrolling." into cm-10.2