[Android] Adjust the parameters for extension's two methods for reflection.

The onMessage and onSyncMessage are changed due to the support for multiple
instances for extension mechanism. However, the Java extension mechanism
doesn't change its parameters when looking up the methods for external
extensions. The unit test for external extensions will be added soon.

BUG=
Merge pull request #978 from yongsheng/multiinstance

[Android] Adjust the parameters for extension's two methods for reflecti...