Update SupervisionIntentProvider#getSettingsIntent to return null if supervision is disabled.

This fixes a bug where the implementation did not match the documented
and expected behavior.

Bug: 438120434
Test: atest SupervisionIntentProviderTest
Flag: android.app.supervision.flags.enable_supervision_settings_screen

Change-Id: I3544437240af361988471e8ea00d0c65009bfe51