Do Android build in CI

[gnprice]

Fixes #772.

(Started as a draft PR, because I wasn't sure if more setup was needed in ci.yml in order for this build to work. Indeed some was.)

[chrisbobbe]

Thanks, LGTM! I'll hold off merging until @rajveermalviya has a chance to look.

[gnprice]

The other day I remarked:
#718 (comment)
that when we handle #772, we should include running the linter. It turns out that the type of issue that we were discussing there was a warning from the Kotlin compiler — so the last commit in this PR, telling the Kotlin compiler to treat those warnings as errors, does the job.

There's also an Android linter, but it has a different job. I'll send a separate PR for running that.

[rajveermalviya]

Thanks for working on this @gnprice, LGTM!

I tried this with macOS M1 runner (macos-latest), but surprisingly it didn't make much difference: https://github.com/rajveermalviya/zulip-flutter/actions/runs/9852479999

[gnprice]

I tried this with macOS M1 runner (macos-latest), but surprisingly it didn't make much difference: https://github.com/rajveermalviya/zulip-flutter/actions/runs/9852479999

Thanks for trying that. Yeah, it's a shame that this adds so much to the time CI requires — about 4 minutes. It'd be good to find a way to reduce that, but I'm not sure there will be a good way; I think building an Android app is just slow compared to what we do in flutter test, and the worker machines we get from GitHub Actions are low-powered, and so on net it's slow.