Clear launch root tasks when WM shell is killed

SysUI crashed repeatedly after toggle SystemUIGX when legacy
split-screen enabled. NPE was thrown whenever SysUI restarted
because the WM Core was unable to add child tasks to the
launch root tasks which already being removed from the WM
hierarchy.

Bug: 180581256
Test: toggle SystemUIGX in split-screen
Change-Id: Iede701f8eccfbe726e72273c36bf789e22780d9f