Moved callback initialisation from start() to constructor so it can be

api/net/sf/briar/api/transport/TransportPlugin.java

...et/sf/briar/api/transport/batch/BatchTransportPlugin.java

.../sf/briar/api/transport/stream/StreamTransportPlugin.java

components/net/sf/briar/plugins/AbstractPlugin.java

...nents/net/sf/briar/plugins/bluetooth/BluetoothPlugin.java

components/net/sf/briar/plugins/file/FilePlugin.java

...nents/net/sf/briar/plugins/file/RemovableDrivePlugin.java

...nents/net/sf/briar/plugins/socket/SimpleSocketPlugin.java

components/net/sf/briar/plugins/socket/SocketPlugin.java

test/net/sf/briar/plugins/bluetooth/BluetoothClientTest.java

test/net/sf/briar/plugins/bluetooth/BluetoothServerTest.java

test/net/sf/briar/plugins/file/RemovableDrivePluginTest.java

test/net/sf/briar/plugins/socket/SimpleSocketPluginTest.java