Introduce Multiwindow Fullscreen API

An API to let app launched in default freeform windowing mode request
entering fullscreen windowing mode. This is useful to let apps, such as
video streaming apps, to provide a unified and immersive experience to
their users.

The request will only be approved if the freeform window is not
requested by the user, i.e., the app is launched in default freeform
windowing mode. If the user get the app from recent screen, or the app
is in other multi-window mode involves user interaction, such as
split-screen, the request will be rejected, and a corresponding callback
will be sent to the app.

The app can provide an optional consumer to receive callback from the
system to learn about the result of the validation.

When the WM core approves the request, a shell transition will be
initialted and the shell will have control of the final result and the
animation of the transition. When shell transition is not enabled, a
direct windowing mode change will be applied.

Bug: 222454650
Test: atest CtsWindowManagerDeviceTestCases:FreeformWindowingModeTests
Change-Id: I921e230ad08b0b2ce6df9e2ad6e01377cb459f3d