appops: do not prune apps that are not present

Do not prune apps that are not currently present in the device (like sdcards ones). During
booting sdcards are not available but must not be purge from appops, because they are still
present in the android app database.
JIRA: CYAN-2811
Issue: https://jira.cyanogenmod.org/browse/CYAN-2811

Change-Id: I6680cbdf0022812b45d966dffee754399e92accb
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>