[Panlingual] Use feature flag to switch opt-in on/off

 - Currently per app language use opt-out by default. This change is to
   add a new idea to have a way to change opt-out to opt-in, and let
   user be able to use LocaleConfig.xml to control the feature more
   precise.
 - If app does not support locale picker or there is no locale provided, remove entries' UI.

Bug: b/231396734
Bug: b/230688538
Test: local
Change-Id: I2661fffab804a2816744711130b26aa2ec47f820