Platform Views: Android and MethodChannel APIs, and implementation sk…

…eleton.

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.

shell/platform/android/io/flutter/plugin/platform/PlatformViewsController.java

@@ -4,13 +4,11 @@
 
 package io.flutter.plugin.platform;
 
-import android.util.Log;
 import io.flutter.plugin.common.MethodCall;
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.StandardMethodCodec;
 import io.flutter.view.FlutterView;
 
-import java.util.HashMap;
 import java.util.Map;
 
 /**

travis/licenses_golden/licenses_flutter

@@ -202,6 +202,11 @@ FILE: ../../../flutter/lib/ui/window/window.h
 FILE: ../../../flutter/shell/common/skia_event_tracer_impl.cc
 FILE: ../../../flutter/shell/platform/android/apk_asset_provider.cc
 FILE: ../../../flutter/shell/platform/android/io/flutter/plugin/common/JSONUtil.java
+FILE: ../../../flutter/shell/platform/android/io/flutter/plugin/platform/PlatformView.java
+FILE: ../../../flutter/shell/platform/android/io/flutter/plugin/platform/PlatformViewFactory.java
+FILE: ../../../flutter/shell/platform/android/io/flutter/plugin/platform/PlatformViewRegistry.java
+FILE: ../../../flutter/shell/platform/android/io/flutter/plugin/platform/PlatformViewRegistryImpl.java
+FILE: ../../../flutter/shell/platform/android/io/flutter/plugin/platform/PlatformViewsController.java
 FILE: ../../../flutter/shell/platform/darwin/desktop/Info.plist
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Flutter.podspec
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Info.plist