Move exodus to GitHub action, and analyse release version instead of debug version

[bmarty]

Currently exodus check is performed by buildkite: https://github.com/matrix-org/pipelines/blob/master/element-android/pipeline.yml#L62 on release and debug version of the app.

This as to be done by GitHub Action now, and could later be removed from Buildkite pipeline. As a reminder, we keep Buildkite as a CI Backup for now, and it's still used to build the unsigned Gplay released version of the app.

Other things to change when implementing this issue:

• exodus should be run only on the Release build. On Debug build we can have other trackers, and this is fine (I think). This is blocking Setup Flipper #6300, see Setup Flipper #6300 (comment) .
• it means that GitHub action should build Release version of the app on every PR. Currently this is not the case, and this is a problem, since Debug builds can succeed and Release builds may fail. Some code/dependencies are different for both types.
• Bonus: it will fix this TODO

[michaelkaye]

#6308 is a first attempt at this.