Fix introduction after one was declined

When we received a remote decline we always went into the REMOTE_DECLINED state
while there's two cases where we need to go into the START state instead.
So when the new request arrived, we weren't in START and thus aborted the protocol.
This commit fixes this.

Fixes #1516

briar-core/src/test/java/org/briarproject/briar/introduction/IntroductionIntegrationTest.java

@@ -405,6 +405,51 @@ public class IntroductionIntegrationTest
 		assertFalse(listener2.aborted);
 	}
 
+	@Test
+	public void testNewIntroductionAfterDecline() throws Exception {
+		addListeners(false, true);
+
+		// make introduction
+		long time = clock.currentTimeMillis();
+		introductionManager0
+				.makeIntroduction(contact1From0, contact2From0, null, time);
+
+		// sync request messages
+		sync0To1(1, true);
+		sync0To2(1, true);
+		eventWaiter.await(TIMEOUT, 2);
+
+		// sync first response
+		sync1To0(1, true);
+		eventWaiter.await(TIMEOUT, 1);
+
+		// sync second response
+		sync2To0(1, true);
+		eventWaiter.await(TIMEOUT, 1);
+
+		// sync both forwarded response
+		sync0To2(1, true);
+		sync0To1(1, true);
+		eventWaiter.await(TIMEOUT, 1);
+
+		assertFalse(listener0.aborted);
+		assertFalse(listener1.aborted);
+		assertFalse(listener2.aborted);
+
+		time = clock.currentTimeMillis();
+		introductionManager0
+				.makeIntroduction(contact1From0, contact2From0, null, time);
+
+		// sync request messages
+		sync0To1(1, true);
+		sync0To2(1, true);
+		eventWaiter.await(TIMEOUT, 2);
+
+		assertFalse(listener0.aborted);
+		assertFalse(listener1.aborted);
+		assertFalse(listener2.aborted);
+	}
+
 	@Test
 	public void testResponseAndAuthInOneSync() throws Exception {
 		addListeners(true, true);