[embeddingAPI][XWALK-2376] Add 6 test cases for XWalkPreference

- Add 6 test cases and update 2 test cases for the latest feature in XWALK-2376

Impacted tests(designed|approved): new 6, update 2, delete 0
Unit test platform: [Android]
Unit test result summary: pass 8, fail 0, block 0
Merge pull request #900 from zhuyongyong/yongzhu/embeddingAPI-tests/XWalkPreferences

[embeddingAPI][XWALK-2376] Add 6 test cases for XWalkPreference