Add a MAC to the ACTIVATE message to prevent the introducer to fake them

A fake ACTIVATE message would cause us to activate the transport keys
before the contact has received our auth message,
which would compromise forward secrecy.

briar-api/src/main/java/org/briarproject/briar/api/introduction/IntroductionConstants.java

@@ -26,4 +26,7 @@ public interface IntroductionConstants {
 
 	String LABEL_AUTH_NONCE = "org.briarproject.briar.introduction/AUTH_NONCE";
 
+	String LABEL_ACTIVATE_MAC =
+			"org.briarproject.briar.introduction/ACTIVATE_MAC";
+
 }