Clear member reference AR#mStartingWindow once starting window death.

Clear the local reference of starting window once the window is death,
so it won't be reuse anywhere else.

Bug: 308334422
Test: launch test app to show splash screen then kill systemui
immediately, verify the starting window reference can be removed right
after receive binderDied.

Change-Id: Ic1b35d9a05d761e9716e3d5f603847df1d7b4d26