Add DevicePolicyEngine::getEnforcingAdminsForPolicy

The new method gets the EnforcingAdmins for the given policy definition.

Test: atest FrameworksServicesTests_devicepolicy:com.android.server.devicepolicy.DevicePolicyEngineTest
Bug: 414733570
Flag: EXEMPT code not used yet

Change-Id: I848f0ac9d8fc1949e1e502f520baf7a06d77cadb