Change Transition

This adds a new transition classification for apps which are
switching between 2 visible states. The primary example of this
is windowing-mode change.

This new classification is mChangingApps in DisplayContent and
there is a transition type for TASK_CHANGE_WINDOWING_MODE.

The mode-change transition is scheduled when AppWindowToken
detects a windowing-mode change. It immediately creates
a snapshot and temporary leash to "freeze" the visuals of
the app until the animation starts (all transitioning apps
are ready).

The transition itself is a blend of fading, scaling, and
cropping based on whether the end-state is bigger than the
start state.

The RemoteAnimationTarget now includes a startBounds and an
extra "snapshot" leash for controlling the snapshot overlay
relative to the AppWindowToken. If either of those is
populated, the client of the RemoteAnimation can assume it
is animating a change and is responsible for transforming
the leash from the startBounds to its final state.

Bug: 113253723
Bug: 111762928
Test: go/wm-smoke + RemoteAnimationControllerTests
Change-Id: I8654656124b888950e55e31d88ce7a4101e935f8