Add back nav animation with minimizing.

- Introduce `DesktopBackNavigationTransitionHandler` for animating back navigation
  transitions.
- Use `DesktopMixedTransitionHandler` to delegate pending minimize
  transitions caused by back navigation to this handler.
- Update `DesktopTasksController` and `DesktopTasksTransitionObserver`
  to create pendingMinimizeTransitions on back navigation cases.

Bug: 323384177
Test: atest DesktopBackNavigationTransitionHandlerTest
Test: atest DesktopMixedTransitionHandlerTest
Flag: com.android.window.flags.enable_desktop_windowing_back_navigation
Change-Id: I44da90ab0c4a2c1500faca30c0123f9c989cdf5c