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/SliceDataConverterTest.java

@@ -128,6 +128,7 @@ public class SliceDataConverterTest {
         assertThat(fakeSlice.getSliceType()).isEqualTo(SliceData.SliceType.SLIDER);
         assertThat(fakeSlice.getUnavailableSliceSubtitle()).isEqualTo(
                 "subtitleOfUnavailableSlice"); // from XML
+        assertThat(fakeSlice.isPublicSlice()).isTrue();
     }
 
     private void assertFakeA11ySlice(SliceData fakeSlice) {