Treat fullscreen windowing mode as fillsParent

At this moment not all fullscreen tasks fill its parent. They don't fill
parents because they are in letterbox, but in such cases it should still
let window containers above it know its requested orientation so when
display decides to respect its requested orientation it can rotate right
away before making a configuration resolution.

WindowState#isObscuringDisplay() also calls Task#fillsParent() which
seems to need this change as well.

This patching a hole mentioned in the bug because due to other reasons
the previous fix isn't enough anymore.

Bug: 156423660
Test: When fixed to user rotation is disabled display can rotate right
away.
Test: New test passed.

Change-Id: If09995815a505e885c789d288ca97a8c9e2b0bbd