setAccessibilityHeading for silent notif section

This is an addition suggested by the a11y team.

Change-Id: I17a180f3c701a36197b00621ccbe1369ca96c56a
Fix: 320295324
Test: tested manually that "Silent" is considered a heading by TalkBack
Flag: EXEMPT trivial change
diff --git a/packages/SystemUI/src/com/android/systemui/statusbar/notification/stack/SectionHeaderView.java b/packages/SystemUI/src/com/android/systemui/statusbar/notification/stack/SectionHeaderView.java
index 580431a..969ff1b 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/notification/stack/SectionHeaderView.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/notification/stack/SectionHeaderView.java
@@ -68,6 +68,7 @@
         if (mLabelTextId != null) {
             mLabelView.setText(mLabelTextId);
         }
+        mLabelView.setAccessibilityHeading(true);
     }
 
     @Override