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
Merge "CM11 Themes: Put overlays in their own group." into cm-11.0
Merge remote-tracking branch 'cm/cm-11.0' into HEAD