Add setting for Vold Switchable Pair (2/2)

This will be hidden unless the device has non-emulated internal storage
and an external SD, in which case persist.sys.vold.switchablepair will be
set automaticaly.  This does away with the need for ro.vold.switchablepair
to be set in the build.prop.

Change-Id: Id83fc90d771ca3a7179f2c50d0ae828eb3dbaa9f

Conflicts:
	res/values/cm_strings.xml