Add a new column for slices_index table.

To distinguish public and non-public slices, add public_slice column
to the database so we can return corresponding results based on this
value.

Bug: 141088937
Test: robotests
Change-Id: I05d003875a8be27e5cb735b4814eb86d6dc40174

tests/robotests/src/com/android/settings/slices/FakePreferenceController.java

@@ -41,6 +41,11 @@ public class FakePreferenceController extends BasePreferenceController {
         return true;
     }
 
+    @Override
+    public boolean isPublicSlice() {
+        return true;
+    }
+
     @Override
     public boolean useDynamicSliceSummary() {
         return true;