Fix AccessibilitySettingsTest

**Root cause**
- We can no longer mock the fragment.getActivity method because it's a
  final method.
- We need to use custom shadows to implement the method we use but not
  shadowed in Robolectric. (e.g. ColorDisplayManager's
  isReduceBrightColorsActivated method)

Bug: 300157873
Test: atest AccessibilitySettingsTest
Change-Id: Ia7d039dd2e31e6b4a7b9382827f9006d4133ca1d