Adding the ability to customize the subtitle on an unavailable slice

When a slice is depending on some setting and the setting is off,
it shows "depends on another setting".
Add an new attribute for Preference to customize the subtitle when
a slice is unavailable.

Bug: 118399193
Test: Robo test on com.android.settings.slices, com.android.settings.core
Change-Id: I84a8400295b36abb357e5baf98e9be3a8d6ea897

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

@@ -74,6 +74,7 @@ public class SlicesDatabaseHelperTest {
                 IndexColumns.PLATFORM_SLICE,
                 IndexColumns.SLICE_TYPE,
                 IndexColumns.ALLOW_DYNAMIC_SUMMARY_IN_SLICE,
+                IndexColumns.UNAVAILABLE_SLICE_SUBTITLE,
         };
 
         assertThat(columnNames).isEqualTo(expectedNames);