Pipe all desk to desk transitions...

to DeskSwitchTransitionHandler. When a desk to desk transition is
detected on the MixedHandler delegate it to the correct handler.

Test: atest DeskTransitionObserverTest &
DesktopMixedTransitionHandlerTest
Test: Create 2 desks, switch between them using the taskbar icons,
observe the animation
Bug: 440224276
Flag: com.android.window.flags.keyboard_shortcuts_to_switch_desks

Change-Id: Ib0db7fcfe51f28f37fb5dd23f8b26e072871d1a7