Replace use of Sequoia's backend with a custom key store.

- Sequoia's key store doesn't meet pep's needs (in particular, the
ability to search on a key's user id) and trying to shoehorn pep's
needs onto Sequoia's key store abstractions is just introducing
overhead with no appreciable gain in functionality.

- This patch changes the Sequoia backend to use a local sqlite
database to store the public keys.

 #!/bin/sh

 set -e

 engine_dir="$1"

 mkdir -p "$engine_dir/build-android/include/pEp"

 cd "$engine_dir/src"

 cp *.h "$engine_dir/build-android/include/pEp"