Framework changes for moving applications to external volume

* Add static functions for retrieving external apps volume information
* Determine external apps volume using the 'externalApps' attribute in
  storage_list.xml
* Allow moving applications to an external volume regardless if the
  internal storage is emulated or not
* Check if app moving 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