Advanced storage settings: switch external storage

Depends on http://review.cyanogenmod.org/56515

This adds an "Advanced storage settings" menu item under Settings > Storage
with a new option "Switch external storage". This is otherwise known as the
switch vold pair trick. Enabling the setting remaps primary storage to
secondary and vice versa.

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