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/display/AmbientDisplayAlwaysOnPreferenceController.java

@@ -19,6 +19,7 @@ import android.content.Context;
 import android.content.Intent;
 import android.os.UserHandle;
 import android.provider.Settings;
+import android.text.TextUtils;
 
 import com.android.internal.hardware.AmbientDisplayConfiguration;
 import com.android.settings.R;
@@ -54,6 +55,11 @@ public class AmbientDisplayAlwaysOnPreferenceController extends TogglePreference
         return isAvailable(mConfig) ? AVAILABLE : UNSUPPORTED_ON_DEVICE;
     }
 
+    @Override
+    public boolean isSliceable() {
+        return TextUtils.equals(getPreferenceKey(), "ambient_display_always_on");
+    }
+
     @Override
     public boolean isChecked() {
         return mConfig.alwaysOnEnabled(MY_USER);