To keep the scope of this CL as small of possible I'm leaving the actual
implementation of the platform view mechanics to a following CL.
This CL introduces:
* A PlatformViewsController class which will be responsible for creating,
resizing, and disposing platform views.
* A PlatformViewRegistry which is exposed through the PluginRegistry
and allows plugins to register factories for platform views.
Android plugin code will add support for a new platform view type by
implementing PlatformViewFactory, and registering a factory with the
registry, e.g:
```java
registrar.platformViewRegistry().registerViewFactory(
'webview',
new FlutterWebViewFactory()
);
```
On the Dart side, the framework will ask the engine to create new
platform views by sending a create message over the platformviews method
channel with the unique platform view type id, dimensions, and a unique
id allocated by the framework for the new platform view instance.
The platformviews method channel is also used for resizing and disposing
platform views.
