Don't handle touch event if WindowDecoration is null

When an app crashes while dragging the window, handling the
ACTION_CANCEL touch event will cause a crash due to the WindowDecoration
being null. We should really bypass the touch handling if there is no
WindowDecoration in onTouch.

Bug: 409313582
Test: atest DesktopModeWindowDecorViewModelTests
Flag: EXEMPT bugfix
Change-Id: I71718ce00a43de9308eeaf77e6623d131640e81b