The method used by Commons IO to get the available disk space fails on
Android devices that lack a df binary - use the Android API instead.

We don't need to call HMAC-SHA-384 more than once to get 256 bits of key
material. The code will throw a RuntimeException if CIPHER_KEY_BYTES is
changed in future so that one iteration's no longer enough.

Fixes bug #25. CMAC is used as the PRF. Note that we're currently using
a version of Spongy Castle with a CMAC implementation that's vulnerable
to a side-channel attack - this has been reported and fixed upstream but
we haven't yet upgraded to the fixed version.

This removes the other projects' dependency on the Android API.

Both devices try to make outgoing connections and accept incoming
connections simultaneously. This should lead to faster connection
establishment when there are asymmetrical connectivity problems, such as
devices that are unable to receive LAN multicast packets or make
themselves discoverable via Bluetooth.

See issue #development-tasks/20: some devices can send but not receive
multicast packets.