Simplify Android CI with shared action

[marcprux]

This PR updates the Android test step of ci.yml to use the shared swift-android-action to build and test on Android. In addition to simplifying the workflow, it also speeds it up somewhat because it caches the toolchain, SDK, and Android emulator between runs.

[johankool]

@marcprux Well, I see you've made my own GitHub Action redundant. I am fine with this action replacing this part of the pipeline and my own action too. No need to maintain two actions that are this similar. FWIW, I ran this particular action on my fork already to try it out and it worked successfully.

[stephencelis]

Thanks for this! We lost track of it among our other projects. @johankool Just to confirm you suggest merging and using this action instead of the one you contributed?

[johankool]

@Stephencelis Yes, fine to merge. Some of your other repos actually already use Marc's action.

[Joannis]

@johankool / @stephencelis gentle reminder