Framework changes for moving applications to external SD

* Add static functions for retrieving external SD volume information
* Determine external SD volume using the 'externalSd' attribute in
  storage_list.xml
* Allow moving applications to the SD card regardless if the internal
  is emulated or not
* Check if moving apps to SD is disabled

Thanks to Pierre-Hugues Husson for identifying an issue causing the
SD card state to be incorrect (ie. SD card appearing unmounted when a
USB drive is unplugged)

Change-Id: I5561c2cf3f558b729c89663852173a532994a47a