Recent changes to Andreas Schildbach's Bitcoin client for Android have removed the option to export private keys. This has been replaced by an option to back up the entire wallet. Unfortunately, the wallet data of the Schildbach client is stored in an arcane format called protobuf (short presumably for Protocol Buffers). The Schildbach client can restore a protobuf wallet from its AES-encoded backup file. But the protobuf wallet cannot be imported into a different Bitcoin client such as the Bitcoin Core (QT) client.
For emphasis, I'm aware of the command line method of decoding the back-ups created by Schildbach's client, which in the past allowed the private keys to be saved in AES encrypted format to /sdcard/Download. The following command worked in the past to produce a human readable copy of my Bitcoin private keys:
openssl enc -d -aes-256-cbc -a -in encrypted-wallet-keys -out decrypted-wallet-keys
This no longer works. The following command instead produces a non-AES-encrypted protobuf wallet.
openssl enc -d -aes-256-cbc -a -in bitcoin-wallet-backup-2014-07-19 -out bitcoin-wallet-backup-2014-07-19-decrypted
The -out file is not an ASCII copy of my private keys but a copy of the protobuf wallet used internally by the Schildbach Bitcoin client (found at /data/data/de.schildbach.wallet/files/wallet-protobuf).