OzoneDisplay: Move Window constants to a separate header.

WindowState and type are defined in OzoneDisplay. Moving these to a separate
header helps in explicitly defining the WindowType and state in IPC Messages.
This CL makes changes in this direction.
DRWH: Introduce interface for handling window state change events.

EventConverter handles native events coming from Wayland and dispatches them to
Aura. We need something similar to handle the case where Aura needs to send
native events(i.e. Creating Window, changing it's state etc) to Wayland. We
already have the code which does this but needs some re-arrangement. This CL
introduces WindowStateChangeHandler interface. The actual implementations will
be done in follow up CL.
WaylandDisplay and ODChannelHost to inherit WindowStateChangeHandler.

WindowState notifications need to be sent over IPC in multi process case and
handled in-process in single process case. OzoneDisplayChannelHost implements
WindowStateChangeHandler interface and sends any window state change
notifications over IPC (as it does now). WaylandDisplay implements
WindowStateChangeHandle interface and adds necessary support to handle the
events in GPU (multi-process case) or in Browser process (single-process case).
Take WindowStateChangeHandler into use.

This CL replaces the usage of OzoneDisplay SetWidget* APIS with the ones
introduced by WindowStateChangeHandler.
WindowStateChangeHandler: Remove all unused API in OzoneDisplay.

This CL removes all unused API in OzoneDisplay and DisplayChannelHost. All
these API have been replaced by WindowStateChangeHandler interface.