storage: allow swapping 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

By setting the persistent system property
 persist.sys.env.switchexternal=1
the primary storage device returned by getExternalFilesDir() for
applications will become /storage/sdcard1. This environment variable
change is effective on devices with either emulated or non-emulated
primary storage.

Change-Id: Ie091eb632280c2dbfd00bbf5f9b0b09be0239234