Make window token add/remove APIs require displayId

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