Bitcoin Wallet for Android allows you to back up your wallet. It asks for a password when doing so.
After much faffing, I finally managed to.. dum, dum, dum! .. copy the file to the SD card, and then I removed the SD card and mounted on a Linux system.
According to this question, the way to decrypt it is
$ openssl enc -d -aes-256-cbc -a -in bitcoin-wallet-backup-2016-09-30
but I get:
enter aes-256-cbc decryption password:
error reading input file
when doing so. This is an extremely poor error message from openssl. I have confirmed that the file is readable using other utilities.