Android: Secure Gradle download against MITM

**Describe the bug**
The android Gradle download (?) doesn’t seem to be verified:
FDroid Bot tells us the following:

> gradle/wrapper/gradle-wrapper.properties is missing [distributionSha256Sum](https://docs.gradle.org/current/userguide/gradle_wrapper.html#sec:verification).

> This means that the gradle download is not verified.

> We recommend explicitly setting the expected Sha256sum to protect you and your apps if a bad actor gets access to the Gradle servers or manages to MitM your internet connection.

> Please note that Android Studio does not currently support this and may or may not crash, but we suggest you try our suggestion anyway to improve the security of yourself and your app.

> Here is an example of how to fix this:
```
#Mon Aug 03 13:45:58 PDT 2020
distributionBase=GRADLE_USER_HOME
distributionPath=wrapper/dists
zipStoreBase=GRADLE_USER_HOME
zipStorePath=wrapper/dists
distributionUrl=https\://services.gradle.org/distributions/gradle-6.1.1-all.zip
distributionSha256Sum=10065868c78f1207afb3a92176f99a37d753a513dff453abb6b5cceda4058cda
```

**Additional context**

See this comment on GitLab for more information: https://gitlab.com/fdroid/rfp/-/issues/2011#note_837630418

Next steps:
* Include the proposed fix in the repo (I don‘t know the Android build enough to judge if this has any negative impact).

Beginne mit gradle/wrapper/gradle-wrapper.properties und prüfe die konfigurierte distributionUrl. Sieh in der Dokumentation zur Wrapper-Verifizierung von Gradle nach, um den erwarteten SHA-256-Wert zu bestimmen, füge die Einstellung für die Prüfsumme hinzu und führe den Android-Build aus, um zu bestätigen, dass der Wrapper weiterhin funktioniert.

Vom Indexierungsmodell aus dem Issue-Text verfasst.