Add a new feature flag for the expand button in support lib.

- check the feature flag to determine whether to use the existing
progressive disclosure mixin or use the expand button in the support
library.
- add corresponding preference xml files that add the new preference
group attribute to use with the expand button in the support lib.

Bug: 63985174
Test: make RunSettingsRoboTests
Change-Id: Ida6eb6182a8066ad1413b7f6142512345fd914d0

src/com/android/settings/notification/SoundSettings.java

@@ -109,7 +109,8 @@ public class SoundSettings extends DashboardFragment {
 
     @Override
     protected int getPreferenceScreenResId() {
-        return R.xml.sound_settings;
+        return mProgressiveDisclosureMixin.isEnabled()
+                ? R.xml.sound_settings : R.xml.sound_settings_new_advance_button;
     }
 
     @Override