Introduce DesktopPlatformScreen to handle platorm specific stuff in Screen.

This patch adds DesktopPlatformScreen. Idea is eventually Ozone implementations
need to only implement this class to provide any platform specific support.
Remove Setter/Getter functions from EventConverterInProcess.

Currently, EventFactoryOzoneWayland and EventConverterInProcess knows
about all observer instances (i.e. windowchange, outputchange etc). This
patch makes changes so that EventConverterInProcess doesn't have this
knowledge anymore and queries the same from EventFactoryOzoneWayland.
Add support in EFWayland to Set/Get WindowStateChangeHandler.
Move ImeStateChangeHandler Getter/Setter to EventFactoryWayland.

This patch moves ImeStateChangeHandler getter and setter to
EventFactoryWayland.
Add support in EventFactoryWayland to release StateChangeHandler.

Currently OzoneChannelHost will create RemoteStateChangeHandler, which
is responsible for sending IPC calls related to Window events. In
single process mode we don't need this. This patch adds support in
EventFactoryWayland to release RemoteStateChangleHandler as needed.
Move WindowTreeHostDelegateWindow to platform/

WindowTreeHostDelegateWindow should be only used by OzoneWalandWindow as
WindowManager and has no dependencies on DesktopAura. This patch moves the
files to under platform/ and don't compile it anymore as part of DesktopAura.
Rename WindowTree*Wayland as WindowManagerWayland.