Visual indicator of transition to fullscreen

First step in providing animations for transition from freeform to
fullscreen. Blue transparent indicator will appear behind a dragged task
when ending the task in that position will result in a transition to
fullscreen.

Bug: 259278770
Bug: 266973675
Test: Manual, drag desktop mode tasks to taskbar, ensure visual
indicator and animation both appear correctly.
Test: atest WindowDecorationTests, DesktopModeControllerTest,
DesktopModeWindowDecorViewModelTests

Change-Id: Ia2308a61092262f719154e255d86c11fe5901123