Clear identity for SupervisionManager check.

Previously the isSupervisionEnabled call was being called with app
identity. This resulted in broken tests. This commit fixes by clearing
the identity before using the manager.

Flag: android.content.pm.protect_supervision_packages
Bug: 408407211
Test: atest FrameworksMockingServicesTests:com.android.server.pm.ProtectedPackagesMockedTest
Change-Id: I50c5ac7e7bf33e5ca7e3467297f1a3287192c917