Fixes an issue where the notification header could be invisible

It's not actually necessary to set the visibility of the notification header
since it is actually in the ChildrenContainer which already handles the visiblity.
This existed from legacy times where it was outside of the viewgroup.
When animating the headers, this could actually lead to blank headers.

Test: runtest -x packages/SystemUI/tests/src/com/android/systemui/statusbar/ExpandableNotificationRowTest.java
Change-Id: I7763b47340a347ed44fc9ce34a98f74c7e55ef8d
Fixes: 35993698