Cleanup: Compatibility checking in DPMS

Rather than call the IPlatformCompat service directly, use the
CompatChanges class via the Injector.

This reduces boilerplate code dealing with the service and eases
testing as it's easier to fake/mock the injector.

Bug: 165573442
Test: atest com.android.cts.devicepolicy.MixedDeviceOwnerTest#testResetPasswordWithToken com.android.cts.devicepolicy.MixedDeviceOwnerTestApi25#testPasswordRequirementsApi
Change-Id: Ieb29f87a4a09910cd5f710cd1e0ddd4b62e808e4