Add tests for gobal and local policies in DPE

Test the following behaviours:
- Global policies are set correctly
- Setting a local policy twice calls the resolution
- Local policies override global policies
- Global policies do not override local policies
- Setting global or local policies with the wrong funciton fails

Test: atest FrameworksServicesTests_devicepolicy:com.android.server.devicepolicy.DevicePolicyEngineTest
Bug: 416180519
Flag: TEST_ONLY