Support SDK installation from command-line

[Ayowel]

At the moment, there is no command-only way to install the SDK from the command-line, which is an issue when automating Ren'Py games' releases as it makes it impossible to 'cleanly' automate Android releases on CI/CD platforms.

This PR:

• Fixes an issue with the inserted buildlib path when running from a directory that is not the one that contains android.py
• Removes unused imports
• Makes it possible to run lib/py?-[...]/python -m rapt.android from Ren'Py's directory to install the android SDK from the command-line on a Ren'Py install

[do10HM]

This is absolutely a step in the right direction but I feel like we should also be able to just use an env variable for the SDK path. I develop for other android engines; why do I need a separate SDK directory just for ren'py?

[wbrambley]

You don't need separate android SDK, just create symlink for SDK folder after installing rapt.