Run Android build in CI

[gnprice]

This would help us more quickly catch issues where the build gets broken, like:

• Android build fails on Flutter main, in flutter_local_notifications #771

It'd also be helpful for when we're making changes to our Android-side code (including our Kotlin source code and our Android build setup).

Once we have this in our own CI, we should also (and I'll file a separate issue for this) see about putting it into Flutter's customer tests (like we've already done with flutter analyze and flutter test, as #239). That way, an issue like #771 would be caught at an earlier stage — the PR that made that change in the Flutter repo would have already seen a test failure. At that point, running it in our own CI on Flutter upgrades should be a bit redundant — but it'll still be valuable for upgrading other dependencies, and for our own changes.