Fixed a bug where the low-priority header wasn't cleared

For notification groups, the header could not be removed
properly leading to overlapping text.

Test: runtest -x packages/SystemUI/tests/src/com/android/systemui/statusbar/stack/NotificationChildrenContainerTest.java
Change-Id: I6823a6137655e9266e46eb33dc33c0ceaee1f45c
Fixes: 36006673