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.
You may use PortDB.apk and export tables to CSV. I sync the export folder with MEGASync, my journal file is acessible by 3 devices (KaliLinux, Windows7 and Android) https://apkpure.com/br/portodb-database/com.portofarina.portodb