Fixed issue with associating WindowToken with null binder.

App don't have to specify a LayoutParams.token when adding a window to
the system, however when they don't WM maps all the windows to a single
WindowToken mapped to a null IBinder. This isn't correct since the
windows can be coming from different apps and also null binder shouldn't
be used to map tokens.
We now:
1. Associate the WindowToken with the IWindow client for bookkeeping in
WM if the app didn't specify LayoutParams.token for the window it is
adding.
2. Throw an illegal argumenet exception we we try to associate a null
binder with a window token or null window token with a binder on a
display.

Fixes: 38021710
Test: Start an alert window, lock and unlock the phone, long press to
bring up power dialog, and tap outside it to make sure it goes away.

Change-Id: I6816b7fb9b9a0a8f5387062bada862eb75004e4f