Do not show window decor for PIP

DesktopModeWindowDecorViewModel#shouldShowWindowDecor should show false
when windowing mode equals WINDOWING_MODE_PINNED

Test: swipe up from fullscreen youtube video to enter PIP. There should
not be black borders
Bug: 287350098

Change-Id: Id1c75e8a60c2e2923d5d5132c9ed263635a3a84d