Set task surface crop to null when in fullscreen mode

Even though the task width & height is correct, setting the window crop
to that instead of null causes input events in some regions of the task
surface to not be sent to the app window.

Bug: 300324920
Test: with desktop windowing enabled, launch YT in fullscreen, verify
touch (e.g. scrolling) works on all parts of the window.

Change-Id: I908debc8d457625b447289913dedc62d4559b3b5