Add top-level Communal settings.

This change adds a new top-level setting, of which the availability is
controlled by a build time config value. It also registers the new
communal category so that prebuilt packages can inject preferences into
it.

Bug: 261641080
Test: verified on device that communal settings show up on top level
Test: atest ScreenSaverPreferenceControllerTest
Change-Id: Idf79ae5b89ecc3498373de56a677b4876fb121c3