Add emulator support

Hi there,

I would like to be able to make assertions on android emulators available on the system.
I plan to add a config that allows me to

- specify that there should be an emulator with a given name
- specify that there should be an emulator with an sdk version
- a mix of the above

I have a couple of questions before starting to work on this:

- Would it make sense to extract android stuff and make this co-dependent on the other module? It seems like android devs could benefit from it as well.
- Do you think an important property of emulators is missing?

The issue names no files, tests, or entry points, so first inspect the existing configuration and assertion behavior before deciding where emulator support belongs. Clarify the module boundary and required emulator properties; done means assertions can verify an emulator by name, SDK version, or both.

Written by the indexing model from the issue text.