vold: allow swapped storage paths at vold init

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

By setting persistent system properties
 persist.sys.vold.switchexternal=1
 persist.sys.vold.switchablepair=sdcard0,sdcard1 (for example)
two non-emulated extended storage devices can swap between primary
and secondary storage.

This commit is heavily inspired by Rick Fernie and Ricardo Cerqueira's
"vold: Allow pre-configured device pairs to switch mountpoints"
[http://review.cyanogenmod.org/12764] but adapted to CM's newer vold.

Change-Id: I2be5d4799c2d8f04a535096ac4c7619849f1f84a