Add handling for main Supervision settings toggle.

This change introduces the basic behavior for the main toggle switch to
enable supervision. There is currently no PIN setup dialog when the
toggle is clicked. That will be added as a follow-up.

Intake bug: b/379312924

Change-Id: I75d6b2de1a7234c6da26fcdcf1d933eed3106a8c
Test: atest SupervisionMainSwitchPreferenceTest
Bug: 392694561
Flag: android.app.supervision.flags.enable_supervision_settings_screen