Rename internal message callback funtion names.

For interface XWalkExtensionAndroid, we think 'onMessage' is a better
name than 'handleMessage', so rename it. So does for handleSyncMessage.

Note, XWalkExtensionAndroid is internal only, so won't affect any
existing android extensions, interal ones and external ones.
Add extension support for embedding API.

BUG=XWALK-1052