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

Requires http://review.cyanogenmod.org/#/c/44585/

Change-Id: I42aebc9fc0d3530cf0a3efb4d2139b688707d43d
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>

Conflicts:
	res/values/strings.xml