Storage Settings: Allow user to trigger a volume rescan

This change allows to trigger a rescan of a volume with the next restrictions:
- Internal volumes if they are emulates
- Primary volumes
- Non-Removable volumes

Ported from: http://review.cyanogenmod.org/44586

Change-Id: I7e75a324938d8081fd715f702d50f1fdceabd96d
Signed-off-by: scott <scott@ubuntu>

Conflicts:
	res/values/cm_strings.xml