AppOps: Do not prune apps that are not present

Do not prune apps that are not currently present in the device (like
USB memory ones). While booting, they are not available but must not
be purged from AppOps, because they are still present in the Android
app database.

Issue-Id: CYAN-2811
Change-Id: I6680cbdf0022812b45d966dffee754399e92accb
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>