Private messages are now the same as group messages, but groups can be
private or public. When a contact is added, a private group is created
and designated as the inbox for exchanging private messages with the
contact.

This produces a saner user experience when devices have differing
clocks.

This allows quick access to any part of the app, including quitting (two
touches).

This makes the contact list behave how alpha testers expected: touching
a contact shows private messages exchanged with the contact.

Removed Commons IO, which we were only using as a replacement for
File.getFreeSpace() on desktop plaftorms.

Note: The Huawei U8210 (Android 2.1) doesn't have all the Java 1.6
standard library methods, and crashes if they're called. Specifically,
String.isEmpty() and NetworkInterface.supportsMulticast() are missing,
so the changes removing those methods were not reverted.

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.

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

See issue #3612607.

Both sides now close the connection after exchanging confirmation
results unless both results are positive.

Some Android devices require a lock to be held while using multicast, to
disable the packet filter that normally filters out multicast packets.

Fixes issue #3611924.