CM11 Themes: Put overlays in their own group.

Overlays will now be added to their own group and the
target group will contain a pointer to the overlay
package to be used in getResource and getBagLocked.

Change-Id: Icc6c6eb791f04966e68dbc099eae8e7129546414
(cherry picked from commit 4c8a2c4784e44a28fbb5247e92a4ab8ec27485da)