CI: add demo-projects-android build

[noomorph]

The demo projects are tested solely in Mac+iOS chain on CI:
https://jenkins-oss.wixpress.com/job/detox-demo-projects-pr/

To achieve parity, we might add a Linux+Android build for demo projects on CI as well.

[LeoNatan]

What is the point of having the demo projects run in CI at all? They just take time and are useless. The main test suit already runs and displays viability.

[noomorph]

1. They serve as a good starting point for anyone who wants to demonstrate a Detox+iOS or Detox+Android issue to us because they are very minimalistic and simple. The example projects are almost empty and don't have a mental overhead that detox/test does. I saw recently the people used them, e.g. in Detox with react-native-navigation build is failing for react-native version over 56. error is recreated in the detox-example project #1137 .

2. Demo projects can become a basis for quick smoke tests, to check for any foreseeable blocker issues with newly arriving React Native, XCode, and Android SDK versions. Since their functionality is minimal, they can signalize about first install&run issues - will it blend? so to say.

[LeoNatan]

I'm not saying we should remove them. They are good examples of how to setup an app with Detox. But I don't see the value of running them in CI and blocking builds with them. The test suite is already running anyway, in several RN versions, so that gives enough sanity. The added redundancy in this case adds no redundancy.

[noomorph]

Resolved by @d4vidi . Kudos!

[d4vidi]

Thanks @noomorph :) Indeed, as a part of the work I did for #1323.

@LeoNatan you're right in a sense but in this particular context it's a good way to have sanity over the creation of the aar etc. The overhead is pretty low.