[Android][Extension] Fix that crosswalk crashes by can't create jni local ref

It's caused by android extension never release the jstring as
the message posted to Java side.

Fixed it by using ScopedJavaLocalRef for the jstring.

BUG=XWALK-1189
Merge pull request #1700 from wang16/ext_local_ref

[Android][Extension] Fix that crosswalk crashes by can't create jni loca...