[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

tests/robotests/src/com/android/settings/biometrics/face/FaceSettingsFooterPreferenceControllerTest.java

@@ -32,6 +32,8 @@ import android.hardware.face.FaceSensorPropertiesInternal;
 import android.hardware.face.IFaceAuthenticatorsRegisteredCallback;
 import android.os.Looper;
 import android.os.RemoteException;
+import android.platform.test.annotations.DisableFlags;
+import android.platform.test.flag.junit.SetFlagsRule;
 
 import androidx.preference.Preference;
 import androidx.preference.PreferenceManager;
@@ -60,6 +62,8 @@ import java.util.List;
 public class FaceSettingsFooterPreferenceControllerTest {
     @Rule
     public final MockitoRule mMockitoRule = MockitoJUnit.rule();
+    @Rule
+    public final SetFlagsRule mSetFlagsRule = new SetFlagsRule();
     private static final String PREF_KEY = "security_face_footer";
     @Mock
     private FaceManager mFaceManager;
@@ -140,6 +144,7 @@ public class FaceSettingsFooterPreferenceControllerTest {
     }
 
     @Test
+    @DisableFlags(com.android.settings.flags.Flags.FLAG_BIOMETRICS_ONBOARDING_EDUCATION)
     public void testString_faceClass3() throws RemoteException {
         setupHasFaceFeature();
         displayFaceSettingsFooterPreferenceController();