[usecase-embeddingapi] Add readMe.md for embeddingAPI usecase

- Improve MultiSurfaceViewsActivity, MultiTextureViewsActivity and XWalkPreferencesActivity
- Add UsecaseDesign_README.md for embeddingAPI usecase

Impacted tests(approved): new 0, update 3, delete 0
Unit test platform: [Android]
Unit test result summary: pass 3, fail 0, block 0
For the `*.java` code changes, better in another pull request rather than together the the readme change.