Remove legacy transition code from TaskDisplayArea

- Now the background color of animation is handled by
  DefaultTransitionHandler#addBackgroundColor.
- The moveRootTaskBehindBottomMostVisibleRootTask is only used
  by deleted legacy RecentsAnimation.

Bug: 365884835
Flag: EXEMPT remove unused code
Test: CtsWindowManagerDeviceWindow
Change-Id: I820674e3bcde4ec44b78474ff68a6c1ea70c6937