Implement tests for android debug and release builds.

1. Build Android native app with release certificate
2. Build Android native app in debug mode
3. Build Android Cordova app with release certificate
4. Build Android Cordova app in debug mode
5. Build Android Ionic 2 app in debug mode
6. Build Android React Native app in debug mode
7. Verify that .apk file is downloadable
8. Nice to have (create follow-up task if not implemented):
   • verify that release .apk was signed with provided certificate
   • build android app in debug mode for second time and verify that .apk from both builds have been signed with same certificate

Test suite repo - https://github.com/aerogear/digger-test

Please add any relevant information to the README so that these new tests are visible.