Add a setting for allowing overlays on Settings app

In the past, we allowed non system overlays on user-debug rom.
Acoording to the user-debug guildline, we should not have differnt
behavior between the user build and user debug rom.

To maintain the consistency between user and user debug rom,
We're creating a new developer setting for allowing non-system overlay on
Settings. By default, we don't allow any non-system app overlays on
Settings app unless user turns it on explictly.

Test: Run robotest. Turn on setting, see the overlays on Settings.
Turn off setting, do not see the overlays on Settings.
Fix: 144989059

Change-Id: I87f00a2eda91de003c6e542e7ec45a066f46fbf7