Check if the keyboard is active from the Webview plugin

* Unity's TouchScreenKeyboard.visible always returns false on Android when the keyboard is active from the plugin.
* With this fix, you'll be able to know when the soft keyboard has been opened in Android.
* For other platforms, it will just default to Unity's TouchScreenKeyboard.visible
Merge pull request #14 from endavid/feature/isKeyboardVisible

Check if the keyboard is active from the Webview plugin