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

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

@@ -113,6 +113,8 @@ class SlicesIndexer implements Runnable {
             values.put(IndexColumns.SLICE_TYPE, dataRow.getSliceType());
             values.put(IndexColumns.ALLOW_DYNAMIC_SUMMARY_IN_SLICE,
                     dataRow.isDynamicSummaryAllowed());
+            values.put(IndexColumns.UNAVAILABLE_SLICE_SUBTITLE,
+                    dataRow.getUnavailableSliceSubtitle());
 
             database.replaceOrThrow(Tables.TABLE_SLICES_INDEX, null /* nullColumnHack */,
                     values);