Update build artifact workflows & repository messaging

[BenHenning]

Explanation

This PR does two things:

• It updates repository messaging to indicate multiple team members may be unavailable until early January (which will show on all new/reopened PRs once this is merged). I'll update existing PRs so that authors are aware.
• It updates the build tests CI workflow to only upload the Oppia dev APK (as this is the only one practically useful for developers since the AABs can't be directly installed on a device).

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

For UI-specific PRs only

N/A -- Infrastructure-only change.

[BenHenning]

Changes to the build_tests check can be seen here: https://github.com/oppia/oppia-android/actions/runs/3746263462 in the artifacts list (it's only uploading the one APK now).

Note that the repository messaging can't be tested until after the PR is merged, but I had a PR recently close so I'll reopen that to verify that it's working.

Finally, the failing CI check is just a flake that I'll re-run.

[BenHenning]

@seanlip PTAL.

[seanlip]

Thanks! LGTM.

[oppiabot]

Unassigning @seanlip since they have already approved the PR.