[Android][Extension] 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.
[Android][Extension] Add extension support for embedding API.

BUG=XWALK-1052