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/SliceDataConverterTest.java

@@ -125,6 +125,8 @@ public class SliceDataConverterTest {
         assertThat(fakeSlice.getSliceType()).isEqualTo(SliceData.SliceType.SLIDER);
         assertThat(fakeSlice.isPlatformDefined()).isTrue(); // from XML
         assertThat(fakeSlice.isDynamicSummaryAllowed()).isTrue(); // from XML
+        assertThat(fakeSlice.getUnavailableSliceSubtitle()).isEqualTo(
+                "subtitleOfUnavailableSlice"); // from XML
     }
 
     private void assertFakeA11ySlice(SliceData fakeSlice) {