Update DP engine tests to include tests for policy persistence

This new tests includes tests to validate the following:
 * Policy values persist after DevicePolicyEngine restarts.
 * Policy values are no longer present if their persistent data are deleted.

Also, a bit of refactor was done to reuse some setup code, to improve readability.

Test: atest FrameworksServicesTests_devicepolicy
Bug: 416180519
Flag: TEST_ONLY
Change-Id: I1bb20cd739ed6a57ed8c52ce8e024e928e1e166b