Add unit test for DevicePolicyEngine

Write a small class that adds initial setup and get/set test for DPE.
Focus on mocking out as little as necessary to make the tests less
brittle and more useful.

Also add a subtarget FrameworksServiceTests_devicepolicy to speed up
local build and run time. On my cloudtop it cut build time from 16m ->
4m for a small change in the test file. The new target can later be used
to run tests in presubmit.

Test: atest FrameworksServicesTests_devicepolicy:com.android.server.devicepolicy.DevicePolicyEngineTest
Bug: 416180519
Flag: TEST_ONLY
Change-Id: If15415ff0950708ec2e9e9b4aeb1a264751b7cd1