Each platform view created (by a plugin supplied factory) is attached to
a virtual display.
The virtual displays are controlled by VirtualDisplayController objects.
The PlatformViewsController maintains a mapping from a platform view's
id to its VirtualDisplayController, which allows it to operate on the
virtual display for a given platform view ID when asked so over the
method channel.

This is using API level 20 APIs, on lower API levels all platform views
method channel calls are noops.
We can make this work on API 19 with some refactoring to the
TextureRegistry (allow the engine Java code to recycle a texture entry
id).

This CL also adds a platform view id parameter to the
PlatformViewFactory#create() method. This allows plugins to route
platform channel messages to specific instances of a platform view.

TBD in future CLs:
  * Forward touch events to the platform views.
  * Support accessibility for platform views.

flutter/flutter#19030