Associate WindowToken object with only one display at a time

WindowTokens were global objects that contained windows that could
be on multiple displays. This model does not work with the
WindowContainer hierarachy as children (window tokens) can not have
mulitple parents (displays).
We now:
- Track the mapping of binder tokens to window tokens per display
instead of globally . So, you can have a binder token map to
individual WindowToken objects per display.
- WMS.addWindowToken is used to create a WindowToken that clients
can then later add windows to. However, when addWindowToken is called
we don't know the display the client(s) would like to add window to.
So, we track binder tokens that we are allowed to add window for in
the RootWindowContainer and create a window token for the binder on
a specific display when we try to add a window.

Bug: 30060889
Test: Manual testing and existing tests pass.
Change-Id: I81a52a32b01c33ed32169d2da0506b688ea9bc8a