Introduce a new method in SupervisionManager to check if supervision is...

Introduce a new method in SupervisionManager to check if supervision is enabled for a specific user.

I also fixed a NPE caused by accessing DPM internal when null.

Bug: 382038943
Test: atest SupervisionServiceTest
Flag: android.app.supervision.flags.deprecate_dpm_supervision_apis
Change-Id: I0a3df58851de0f06176d3b59f9be4d621a0d51b7