Move introduction role into IntroductionResponse

.../src/main/java/org/briarproject/bramble/api/Nameable.java

...in/java/org/briarproject/bramble/api/identity/Author.java

.../briarproject/briar/android/contact/ConversationItem.java

...org/briarproject/briar/api/introduction/Introduction.java

...arproject/briar/api/introduction/IntroductionRequest.java

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

.../org/briarproject/briar/api/messaging/PrivateRequest.java

...org/briarproject/briar/api/messaging/PrivateResponse.java

...in/java/org/briarproject/briar/api/sharing/Shareable.java

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

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

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

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