Fix visual issues with splitscreen to desktop transition.

Fixes three minor visual issues with the split to desktop transition:

1 - Modifies getFocusedDecor to always prioritize a currently dragged decor,
fixing an issue where some split -> desktop transitions would hide the
dragged task if they ended on the opposite side of the screen.

2 - Adds onSplitToDesktopAnimation to StageCoordinator to
set stages as not visible once the to desktop animation is complete,
preventing the split divider from briefly showing if we then navigate to
overview.

3- When requesting split, sets the task to WINDOWING_MODE_MULTI_WINDOW
to avoid a task flicker. Then, when the stage listener detects a change,
it will set the windowing mode to UNDEFINED to inherit the mode from
split stage, ensuring the task doesn't use freeform bounds in split
stage.

Bug: 336312668
Bug: 336311310
Bug: 323378048
Test: atest DragToDesktopTransitionHandlerTest
Test: manual
Change-Id: I55634fbe9687cd9a8f433818c8bafe983763356c