Return: Expandable Volume Overlay

The option to customize the expandable volume overlay has been
orphaned since the dawn of time(or SlimKat in this case).
Bring back the setting for the same. The frameworks change is
already in the source.

This also fixed the OnPreferenceChanged keys

PS 2-4: Cleanup and MultiUser

Change-Id: I59d3ac8bfc8ea7ee08e4e2ed006aa432e4a929c4