Fix notification launch animation for groups

This fixes a problem where the z ordering is not correct during the launch animation of a child notification inside a notification group. The launched notification was rendered below other notifications in some cases.

Test: Manual, i.e. posting various types of notifications and analyzing the animations in screenrecordings.
Bug: 246932690
Change-Id: I658b4b3db75da5199b7b7cb416b7d7fabbc75520