I want to use the Ledger command line accounting software. Are there any android apps I can use to generate CSV files of expenses when I'm away from home, which can then be imported into Ledger?
Many financial apps support CSV export. I have been using, and contributing to, Money Manager Ex for Android. It is an Open Source app and supports export to CSV and QIF formats.
I also wrote a Python library for reading the database, so export to CSV can happen outside the app itself, either on a PC or on a mobile device.
There are plenty of tools that will convert CSV, or other formats, to ledger syntax, which you can append to your book file.