[embeddingapi] Merge webapi-embeddingapi-xwalk-tests to embedding-api-android-tests

- Merge webapi-embeddingapi-xwalk-tests to embedding-api-android-tests
- Remove embeddingapi folder
- Remove pack.sh, webapi-embeddingapi-xwalk-tests.spec, inst.sh.apk, tests.xml
- Improve README content

Impacted tests(approved): new 0, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 128, fail 15, block 0