Add google-services.json in ./android/app to make app possible to compile

[emanuelb]

Build instructions include in 3: https://github.com/rsksmart/rwallet/blob/master/README.md#getting-started

3. In order to use firebase messaging on Android, put google-services.json in the android/app.

But empty google-services.json file that is valid json (content {}) will result in error:

* What went wrong:
Execution failed for task ':app:processReleaseGoogleServices'.
> Missing project_info object

so used file form: https://github.com/blockchain/My-Wallet-V3-Android/blob/75ebe718cb99c6d302efb90447f1719973deb93f/scripts/quick_start.sh#L7-L46 (same command to create file, but changed the package_name to com.rsk.rwallet.v2) to avoid error:

* What went wrong:
Execution failed for task ':app:processReleaseGoogleServices'.
> No matching client found for package name 'com.rsk.rwallet.v2'

Thus this issue is to implement mechanism to create the google-services.json like is used in Blockchain quick_start.sh above, or add to readme instructions with enough information for creating a valid file.

[patogallaiovlabs]

Hi @emanuelb !
Thanks for the contribution, as this is an open source project, we encourage you to make a PR with this change that you are suggesting, I personally think is a good idea for a quick start/engagement of new contributors.
Best
Pato

[emanuelb]

The fix I used (file from blockchain app) is probably incorrect (blockchain urls inside) even if it's working (app compiles), the google-services.json file should be minimal with enough information to make it work, it would be better if it's based on the file that used for the release version of the app (the same file or same schema/fields content just different values)