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
Advanced storage: swap primary storage for apps

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 a new preference to Advanced storage settings to swap the
primary storage path used for applications storage. Only
one of the two primary storage swap methods can be active at a time.

This option is currently restricted to a secondary storage device
located at /storage/sdcard1 to prevent swapping with not-so-permanent
devices like usbdisk.

The effect of this option can be verified by going to Settings > Apps
and swiping over to "ON SD CARD". The bar at the bottom of the screen
should have used/free spaces corresponding to /storage/sdcard1.

Inspiration for this storage swap method comes from Martin Meloun.

Change-Id: I5eb31735baeb8e356f94cc1b87fa9618023db8ce