Clear all starting window while unregister organizer.

The starting window view might left in client's ShellTaskOrganizer,
clear all of the starting windows view while unregister task organizer
to preventing those views from left.
Consolidating the remove window methods together.

Bug: 207175662
Test: build/flash
Test: atest StartingSurfaceDrawerTests SplashscreenTests
Change-Id: I93e4862b7f866c7c73d9b3999a74e03784848058