Make `SupervisionService` react immediately to profile owner changes.

With this change, as soon as profile owner is granted to the supervision app, supervision will be enabled in SupervisionService.

I also decided to create a supervision-specific flag to guard these changes instead of reusing the DPM one.

Bug: 377261590
Flag: android.app.supervision.flags.enable_sync_with_dpm
Change-Id: Ib7416c4c6080c8402e10aac49e0a1f0955a954cc
Test: atest SupervisionServiceTest