MediaProvider: get UUID of removable volumes for non-FAT filesystems

Change-Id: I0a66d6c169f3da8fb69af09e543d56df2d84e347
MediaProvider: Allow scan for directories

Patchset 2: Allow only directories of external volumes

Change-Id: I5694f75896749d84b163a1213730eaa4c1e776e6
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>