Ensure OzonePlatformWayland is destroyed at shutdown.

Currently, OzonePlatformWayland is not being destroyed during
shut down. We now ensure that it's destroyed before app exists.
Stop DisplayPollThread in StopProcessingEvents.

StartProcessingEvents starts the thread with options and starts polling.
StopProcessingEvents stops polling for wayland events but doesn't
actually stop the thread. This patch fixes it.