CB-8210 Setup message channel for native -> JS events

- Initialize a message channel for native -> Javascript in the core App plugin
- Change keyboard detection to send events via plugin message channel, instead
  using eval() (i.e. webView.sendJavascript())

Related to: apache/cordova-js#96
Rather than use a static method, it'd be better to use webView.pluginManager.getPlugin("org.apache.cordova.App") to get the instance-specific plugin instance