Fix a11y readout of notification importance buttons

Test: manual with TB
Bug: 134047401
Change-Id: Idab3426a37ed3f25c2cde47e0747ac9a31850f62

src/com/android/settings/notification/ImportancePreference.java

@@ -117,11 +117,13 @@ public class ImportancePreference extends Preference {
             case IMPORTANCE_LOW:
                 mAlertButton.setBackground(unselectedBackground);
                 mSilenceButton.setBackground(selectedBackground);
+                mSilenceButton.setSelected(true);
                 break;
             case IMPORTANCE_HIGH:
             default:
                 mSilenceButton.setBackground(unselectedBackground);
                 mAlertButton.setBackground(selectedBackground);
+                mAlertButton.setSelected(true);
                 break;
         }
         setImportanceSummary((ViewGroup) holder.itemView, mImportance, false);