Add isSliceable API to BasePrefController

Only support explicitly approved Settings Slices,
dictated by controllers which return true for the new
method isSliceable.

Updating the supported settings to a whitelist means that
the method to return all available slices must be updated,
and checking slicability when we index slices.

Test: robotests
Bug: 79779103
Change-Id: Ib2b9690cdd0036b5cc4a1cb846c52bce7c824ab9

src/com/android/settings/gestures/DoubleTapScreenPreferenceController.java

@@ -90,6 +90,11 @@ public class DoubleTapScreenPreferenceController extends GesturePreferenceContro
         return AVAILABLE;
     }
 
+    @Override
+    public boolean isSliceable() {
+        return TextUtils.equals(getPreferenceKey(), "gesture_double_tap_screen");
+    }
+
     @Override
     public boolean setChecked(boolean isChecked) {
         return Settings.Secure.putInt(mContext.getContentResolver(), SECURE_KEY,