Include external storage devices in DocumentsUI.

Include volume UUID in generated document IDs to uniquely identify
volumes over time.  Show volume label to users.  Watch for mount
changes to update available roots.

Bug: 11175082
Change-Id: Ia151bde768587468efde0c1d97a740b5353d1582

Conflicts:
	packages/ExternalStorageProvider/src/com/android/externalstorage/ExternalStorageProvider.java

Signed-off-by: CullenDev <cullendev@gmail.com>