Introduce a hidden API in SupervisionManager to return the package of the...

Introduce a hidden API in SupervisionManager to return the package of the active supervision app to hide away implementation details such as GM Kids Module being temporarily the supervision app while the transfer is happening.

The intent of this API is to be consumed by a SettingsLib utility that will launch the supervision settings page and replace PO dependencies such as this one:
http://ac/packages/apps/Settings/src/com/android/settings/enterprise/EnterprisePrivacyFeatureProviderImpl.java?l=264-279

Bug: 382038391
Flag: EXEMPT unreachable code
Test: atest SupervisionServiceTest
Change-Id: I0115badee8fbceba906ac94d418a2f92e0d79f11