Do not depend on actual WindowManagerService object.

The Activity unit tests do not need a real instance of
WindowManagerService and can instead use a mock.

Test: bit FrameworksServicesTests:com.android.server.am.ActivityRecordTests
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStarterTests
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStackSupervisorTests
Test: bit FrameworksServicesTests:com.android.server.am.ActivityStackTests

Bug: 67629551
Change-Id: I2ab681eb88ebfa7113838dab19a3a1c017132955