feat(A11yHaTS): Add entrypoint for magnification page

Visibility is controlled by the aconfig flag and
SurveyFeatureProvider#isSurveyAvailable, and these can be changed by
magnification function state.

Bug: 380346799
Test: Manual testing on Pixel and non-Pixel devices
Test: atest ToggleScreenMagnificationPreferenceFragmentTest
            ToggleScreenMagnificationPreferenceFragmentForSetupWizardTest
Flag: com.android.server.accessibility.enable_low_vision_hats
Change-Id: I1770d381763be8eba429600f28307b64f6ccf83c