Allow Magnification Settings launch from outside of AccessibilitySettings

Before this CL, Magnification Settings can only be
launched from MagnificationPreferenceController.

This CL initializes FooterPreferenceController if the
parent class does not initialize it from Intent extras.

Bug: 268411305
Test: manual
      make RunSettingsRoboTests ROBOTEST_FILTER=MagnificationGesturesPreferenceControllerTest
      make RunSettingsRoboTests ROBOTEST_FILTER=ToggleScreenMagnificationPreferenceFragmentTest
Change-Id: I954739c39c823ce7fe0daa711743f729d182571b