Window tokens can now only be on one display, so we now require clients that want to add/remove window tokens to specify the display they would like the token to be created on. This simplifies the token handling code in WM and will be useful moving forward for clients that want to add windows to external displays. Test: Existing tests pass Change-Id: I6b2d8d58a913b3624f1a9a7bebbb99315613f103