By default notifications are allowed to be shown on the lockscreen

While we are initializing the policy file from disk, if there is no
such file we still have to allow the notifications to be shown.
Previously we were assuming that it's false if there's no such tag
in the xml.

Test: existing tests pass
Change-Id: Ia76a8f3e0af1b176de6141bec1db4babd4f97e00
Fixes: 119833914