Download and use an Android device skin when creating an Android emulator

[paulproteus]

At the moment, the documentation printed by briefcase run android on how to create a virtual Android device results in an emulator without any "skin".

The skin provides helpful visual context to the user to feel that they're testing their app on a mobile device, not just a small computer screen. See e.g. this pic (source)

See discussion of skins at https://github.com/paulproteus/Python-Android-support-old for info on where to get skins.

After updating briefcase, also update the screenshots in the BeeWare tutorial.