[embedding][XWALK-1052] Leverage developer's sample tests for extension.

- Add extension support for embedding API.

Impacted tests(designed|approved): new 1, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #691 from zhuyongyong/yongzhu/embeddingAPI-sample/add

[embedding][XWALK-1052] Leverage developer's sample tests for extension.