deskclock: take persistent grants of external uris

The new storage api used by the DocumentProvider api only grants temporally the selected uris. This
causes that after a reboot the access to the selected uri is denied. This changes reclaims a persistent
read permission for the selected uri while in use by an alarm.

Change-Id: I3dbdfd1c03ab9d302d559c62ae8311a43ce7b336
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>