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/SlicesDatabaseHelperTest.java

@@ -75,6 +75,7 @@ public class SlicesDatabaseHelperTest {
                 IndexColumns.CONTROLLER,
                 IndexColumns.SLICE_TYPE,
                 IndexColumns.UNAVAILABLE_SLICE_SUBTITLE,
+                IndexColumns.PUBLIC_SLICE
         };
 
         assertThat(columnNames).isEqualTo(expectedNames);