Improve DevicePolicyManagerTest for secondary user

Refactor the DevicePolicyManagerTest to support tests from secondary user.
Remove split system user related logic and add tests for headless system user mode's DO setup.

Bug: 137101239
Fix: 171928524
Test: atest FrameworksServicesTests:com.android.server.devicepolicy.DevicePolicyManagerTest
Change-Id: I523044b9d4e0a231fed27278e39005cdca57b95b
4 files changed