[RONs] Fixes for promoted notification logic

Default for CAN_HAVE_PROMOTED_NOTIFS will initially be false, since the UI features do not exist.  When the UI features are enabled, the default will be true.  The value will not persist until the UI features are enabled.

Change-Id: I7a24581c4ba304e8b318408737d45e043c1fcceb
Flag: android.app.api_rich_ongoing
Flag: android.app.ui_rich_ongoing
Bug: 379335139
Test: atest PreferencesHelperTest
Test: atest NotificationManagerServiceTest