Add FOSS flavor

[papjul]

This PR implements a FOSS flavor (i.e. with no proprietary dependencies) and builds it when a new version is tagged. It's a pre-requisite for #1 although publishing it on F-Droid is not mandatory (some users may want to use the FOSS flavor with the internal updater, or something else like Obtainium). Note that compared to Tachiyomi, the updater can be left enabled this time, if you choose to make reproducible builds (which is the recommended way to publish on F-Droid).

Notes:

• I didn't bother building/testing the FOSS flavor everytime there is a commit or a new PR. The differences between the two flavors are so minor, that I didn't feel the additional build time was worth it.
• I only publish the universal APK for the FOSS flavor in GitHub releases, but I can add other ABIs if you want

[AntsyLich]

FOSS build setup can exist for F-Droid but I don't necessarily see a point of having them on GitHub.

[papjul]

FOSS build setup can exist for F-Droid but I don't necessarily see a point of having them on GitHub.

The point of having them on GitHub is to allow reproducible builds. Basically, you build and sign the release with your key, and F-Droid compares it with their own build. If they match, they will publish your release with your key. This makes the release process safer by making the developer sign the releases (and not F-Droid), while allowing F-Droid to still check for any tampering attempt.

Also:

some users may want to use the FOSS flavor with the internal updater, or something else like Obtainium

[AntsyLich]

FOSS builds can stay in a separate repo then similar to preview/beta

[papjul]

Good idea! Should I revert the changes from the .github directory and let you create the other repo?