Replaced PacketWriter methods with a constant.

briar-api/src/org/briarproject/api/sync/PacketWriter.java

briar-api/src/org/briarproject/api/sync/SyncConstants.java

...core/src/org/briarproject/sync/DuplexOutgoingSession.java

briar-core/src/org/briarproject/sync/PacketWriterImpl.java

...ore/src/org/briarproject/sync/SimplexOutgoingSession.java

briar-tests/src/org/briarproject/sync/ConstantsTest.java

...src/org/briarproject/sync/SimplexOutgoingSessionTest.java