Example success:
https://github.com/WorldHealthOrganization/app/actions/runs/155730242

Affects #1044

Unrelatedly, adds a security check to our gradle wrappers.  Read more here: https://github.com/marketplace/actions/gradle-wrapper-validation

----


### Execution

On every push to `master`, an Android APK will be pushed to the Firebase App Distribution staging environment using the `client-distribute-staging` workflow. Firebase will invite all members of the `every-build` testing group to test each of these builds.

### Setup

1.  [x] (First time) Create [a service account](https://console.cloud.google.com/iam-admin/serviceaccounts/details/104111540645440452578?project=who-myhealth-staging) on `who-myhealth-staging` GCP with `Firebase App Distribution Admin` role.
2.  [x] Create a Key for that service account: Add Key -> Create New Key -> JSON. Save the JSON file locally as `key.json`.
3.  [x] Generate a random passphrase. Store that passphrase as GitHub secret `FIREBASE_APPDEPLOY_STAGING_SVCACCT_PASSPHRASE`.
4.  [x] Encrypt and encode the key using the generated passphrase:

        gpg --symmetric --cipher-algo AES256 --armor < key.json | base64

5.  [x] Store the encrypted key as GitHub secret `FIREBASE_APPDEPLOY_STAGING_SVCACCT_JSON`.
6.  [x] Delete the older key on the service account on GCP.