Android studio migration

Merged with Dev branch for latest builds.

- Added a [sample_api_keys][2] file.
- External libraries moved to [/libraries][1] folder.
- Force add files from dev branch to avoid merge conflicts.

The repo has been well tested.

[1]: https://github.com/mipreamble/barterli_android/tree/android-studio-migration/libraries
[2]: https://github.com/mipreamble/barterli_android/blob/android-studio-migration/barterli/src/main/res/values/sample_api_keys.xml