Actual result: My Message very briefly turns blue but immediatly afterwards becomess grey. Touch to retransmitt" (Text translated in my own words).
I found this out by debugging the Android Account Manager code.
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Transact(Binder.java:446) only provides an account (name and type), a scope, and a callback method, there's no parameter to specify an app or something I could customize in the dev API console. Fact is that when you sign a APK and then ask Google for a OAuth2 token, you have to register your signed app through the dev console.
It's a security measure based on the app package name and the sha1 fingerprint.
In the Play Console go to Release Management - App signing If you opted in to Google Play App Signing, you'll see 2 certificates there along with all of their fingerprints.