Create special case mobile data slice

Build the special case mobile data slice for general use,
and for the Internet Connectivity Panel. The Slice controls
the default data subscription, and links to the subscriptions
page.

Bug: 117804089
Fixse: 119411534
Test: robotests & test app.

Change-Id: I31b524a8fe3182a244d940848090e97250632f39

src/com/android/settings/slices/CustomSliceable.java

@@ -71,7 +71,7 @@ public interface CustomSliceable {
      *
      * @param intent which has the action taken on a {@link Slice}.
      */
-    void onNotifyChange(Intent intent);
+    default void onNotifyChange(Intent intent) {}
 
     /**
      * @return an {@link Intent} to the source of the Slice data.
@@ -90,11 +90,12 @@ public interface CustomSliceable {
     }
 
     /**
-     * Settings Slices which can represent component lists that are updatable by the
-     * {@link SliceBackgroundWorker} class returned here.
+     * Settings Slices which require background work, such as updating lists should implement a
+     * {@link SliceBackgroundWorker} and return it here. An example of background work is updating
+     * a list of Wifi networks available in the area.
      *
-     * @return a {@link SliceBackgroundWorker} class for fetching the list of results in the
-     * background.
+     * @return a {@link Class<? extends SliceBackgroundWorker>} to perform background work for the
+     * slice.
      */
     default Class<? extends SliceBackgroundWorker> getBackgroundWorkerClass() {
         return null;