Minor refactoring.

components/net/sf/briar/transport/ConnectionDecrypter.java

...nents/net/sf/briar/transport/ConnectionDecrypterImpl.java

components/net/sf/briar/transport/ConnectionEncrypter.java

...nents/net/sf/briar/transport/ConnectionEncrypterImpl.java

components/net/sf/briar/transport/ConnectionReaderImpl.java

components/net/sf/briar/transport/ConnectionWriterImpl.java

...onents/net/sf/briar/transport/PaddedConnectionWriter.java

test/net/sf/briar/transport/ConnectionDecrypterImplTest.java

test/net/sf/briar/transport/ConnectionEncrypterImplTest.java

test/net/sf/briar/transport/NullConnectionDecrypter.java

test/net/sf/briar/transport/NullConnectionEncrypter.java