DocumentsUI: Differentiate USBs from SDcards

* Also remove need for context from getPath()

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
DocumentsUI: Allow installation of apps from external storage

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
Merge topic 'docui_update' into kitkat

* changes:
  DocumentsUI: Allow installation of apps from external storage
  DocumentsUI: Differentiate USBs from SDcards