Let IntroductionResponse know if introduction can succeed

...oject/briar/android/conversation/ConversationVisitor.java

...rproject/briar/api/introduction/IntroductionResponse.java

...iarproject/briar/introduction/AbstractProtocolEngine.java

...rproject/briar/introduction/IntroduceeProtocolEngine.java

...rproject/briar/introduction/IntroducerProtocolEngine.java

...arproject/briar/introduction/IntroductionManagerImpl.java

...oject/briar/introduction/IntroductionIntegrationTest.java