[Biometric Onboarding & Edu] Update face settings page

- Added a feature provider for face settings page in FaceFeatureProvider
  for customization
- When face is deleted, disabled the settings buttons instead of hiding
  them.
- Updated new UX style for add/remove face button.

Bug: 370940762
Test: atest FaceSettingsEnrollButtonPreferenceControllerTest
            FaceSettingsFooterPreferenceControllerTest
Test: manual - 1. Enroll face
               2. Go Face Settings page and remove face
	       3. Enroll face again
Flag: com.android.settings.flags.biometrics_onboarding_education

Change-Id: I490e647523eeff2dd1a58aab07f638e3e5e0ffb8