make Labrador for Android installable via F-Droid

[mi-hol]

target F-Droid instead of Google Play Store or just let the users manually install the APK.

my proposal would be:

1. priority: let the users manually install the APK. (completed via [Android] calibration dialog fix #376)!
2. priority: make it installable via F-Droid

Originally posted by @mi-hol in #343

[mi-hol]

Just learned about new Google requirements for developers on https://keepandroidopen.org that are extremly high hurdles for open source projects to publish apps via Google Play Store :(

[EspoTek]

@mi-hol this has always been the case when publishing for the Play Store, but now Google are going to lock down third-party apps in this way too. Either way, we already have a key that we can use for F-Droid if push comes to shove.

[mi-hol]

we already have a key that we can use for F-Droid

@EspoTek Good to hear. Is this key already stored as a secret?

Related to this specific question the wider topic of Artifact attestations came to my mind.
Would GitHub's attestation make sense for our builds?

[EspoTek]

Yep, this signature is stored as ANDROID_PRIVATE_KEY_BASE64 and ANDROID_CERTIFICATE_BASE64, referenced in android.yml.

It looks like the main issue is getting all the packaging and metadata stuff ready here:
https://f-droid.org/en/docs/Submitting_to_F-Droid_Quick_Start_Guide/