Introduce a hidden API in SupervisionManager to set the supervision enabled...

Introduce a hidden API in SupervisionManager to set the supervision enabled state for the current user or a specific user.

The intent of this API is to be consumed by Settings when a user
interacts with the main Supervision setting toggle.

Change-Id: I451e08af74b9819e4644ca64bad80ca55c089208
Test: atest SupervisionServiceTest
Bug: 392694561
Flag: android.app.supervision.flags.enable_supervision_settings_screen