Fix header not set on initialization

Header not being set was causing min priority
groups to skip expansion animation.

Bug: 38436027
Test: runtest -x frameworks/base/packages/SystemUI/tests/src/com/android/systemui/statusbar/stack/NotificationChildrenContainerTest.java
Change-Id: I34307a7e5ecfe7e30d5bf97880b17fedb0d84378
2 files changed