DeskClock: fix alarms database to prepare m1 to m2 transition

Fix the DeskClock database helper to avoid drop all alarms when incrementing from
version 5 to version 6 (http://review.cyanogenmod.org/#/c/31623/).
This will happen on m1 to m2 transition.

Change-Id: I6f01b42cf93701e441a8e3d9894e1ef414ff9327
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>