Find and resolve memory leaks

Memory leaks are super common in Android. Luckily, there's a nice library that can automatically detect them: https://square.github.io/leakcanary/getting_started/

There are a few places I've seen where we hold onto fragment references in other classes, for example. That's almost certainly leaking memory. There's usually a way to engineer it so you don't need to wrap these things in a `WeakReference`, but that can be used as a last resort if we absolutely need a reference to an object with an Android lifecycle.