Allow freeform apps to enter fullscreen using the request fullscreen

mode api from a user-driven event

Stops
ActivityClientController#validateMultiwindowFullscreenRequestLocked from
returning RESULT_FAILED_NOT_DEFAULT_FREEFORM if the mode even the task is
not freeform by default or currently in freeform. Also returns true if
requesting activity is pinned.

Change also fixes updates Activity API documentation.

Bug: 307360894
Bug: 296268915
Test: Test using multiwindow fullscreen api test app (ag/20460053)

Change-Id: Id87eb9c2b952013cdfc64a4e6cc26750cec2ebd3