MediaProvider: Allow scan for directories

Patchset 2: Allow only directories of external volumes

Change-Id: I5694f75896749d84b163a1213730eaa4c1e776e6
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge branch 'cm-10.1' of github.com:CyanogenMod/android_packages_providers_MediaProvider into jb42