Remove mNeedsZBoost

1. Now the transition layer is assigned by
   com.android.wm.shell.transition.Transitions#setupAnimHierarchy.
2. Currently it is only set on last finishing activity of a task, and
   it won't be unset because ActivityRecord no longer runs transition
   animation in WM core.

Bug: 365884835
Flag: EXEMPT remove unused code
Test: CtsWindowManagerDeviceWindow
Change-Id: Iea7a3a8739138cd536b1cfde11040192d8ce155d