Add dark theme custom type into the dark theme settings page

Also fix some test cases.

I'd prefer using Robolectric shadows rather than mocking Android
framework classes. However, I can't do that in this CL due to the
lack of multiple shadow classes support.

Test: make RunSettingsRoboTests -j56
      ROBOTEST_FILTER=com.android.settings.display.darkmode.*
Bug: 215182463
Change-Id: If557d2933927a5dd0fadd3f6db6bb6f0ab7dd5ee