Add DesktopMixedTransitionHandler.startMinimizeTransition

This CL lets DesktopMixedTransitionHandler handle the minimization
transaction and start the launcher fade-in animation when the last
visible task is minimized.

Bug: 390161102
Test: WMShellUnitTests
Test: manual - Minmize the last task in desktop and the launcher shows with animation
Flag: com.android.window.flags.enable_desktop_windowing_exit_by_minimize_transition_bugfix
Change-Id: I24c893cd6b13790ffbd21eafd29177bfc37aa57e