[Android] Add message passing support

XWalkExtensionAndroid.java is the interface to allow implement message
passing based extension written by java.

When new a instance that inherit from XWalkExtensionAndroid.java, it will
create C++ instance of XWalkExtensionAndroid. Then the C++ extension framework
will create XWalkExtensionAndroidInstance on-demond. Thus they will bridge
the messages passed between JavaScript and Java.

XWalkExtensionAndroid is self-registed to existing XWalkExtensionService.
[Android] Add ExtensionEchoTest for message passing unit test
[Android] Support multiple extension instances

WebFrame java interface is not exposed, thus there is no way to
identify which XWalkExtensionAndroidInstance need to be post message.
Leave a FIXME for that.
Merge pull request #513 from halton/android_postmessage

Support Android message passing mechanism