Question: I would love to start playing around with long exposure photography, more specifically light painting.

Sadly I don't have any camera except the iSight built into my Macbook Pro and my mid-range Android phone (*I'm in college. Money doesn't really exist for me*).

Does anyone know of a way to take long exposure photos using either a Mac or a good Android app that can?

Answer: 
Looks like there is an app that allows you to control the exposure and shutterspeed of the iSight:

<http://www.ecamm.com/mac/iglasses/settings.html#settings>
Based on your budget-related clause, I'd guess you don't have a very current phone, either, but just in case:

It's now possible to sideload the Pixel camera app on (nearly) any Android 9 or 10 phone with compatible camera hardware -- this includes night exposures up to one minute, and may (for compatible phones) include astrophotography mode up to several minutes. I know the night photo mode works on my three year old first-gen Pixel, even though it's not officially supported by this app version; I haven't yet been able to test astro mode.

Some phone models require a specially modified version of the camera app, while others don't, so you'll need to search specifically for your model to be sure you get the correct version of the camera app, but at least the night vision mode is now supported on most phones with high quality cameras made in the past 2-3 years.