Advanced storage: vold swap storage paths

Advanced storage settings patch series:
[1/4] vold: allow swapped storage paths at vold init
      http://review.cyanogenmod.org/56515
[2/4] Advanced storage: vold swap storage paths
      http://review.cyanogenmod.org/56516
[3/4] storage: allow swapping primary storage for apps
      http://review.cyanogenmod.org/56950
[4/4] Advanced storage: swap primary storage for apps
      http://review.cyanogenmod.org/56951

This adds an "Advanced storage settings" menu item under Settings >
Storage with a new option "Swap vold primary storage". Enabling this
option remaps primary storage to secondary and vice versa when vold
initializes.

The implementation is a hybrid of David Hacker's "Add setting for Vold
Switchable Pair (2/2)" [http://review.cyanogenmod.org/48390] and Ricardo
Cerqueira's "Storage: Allow user to choose internal storage"
[http://review.cyanogenmod.org/12858].

Change-Id: I9461b65a8d3afb51c3dbe706687b81bd5f8c920c

Conflicts:
	res/values/cm_strings.xml