briar-headless: Next round of review comments

briar-headless/README.md

...main/java/org/briarproject/briar/headless/BriarService.kt

...s/src/main/java/org/briarproject/briar/headless/Router.kt

...g/briarproject/briar/headless/blogs/BlogControllerImpl.kt

...arproject/briar/headless/contact/ContactControllerImpl.kt

.../briarproject/briar/headless/event/WebSocketController.kt

...arproject/briar/headless/event/WebSocketControllerImpl.kt

...briarproject/briar/headless/forums/ForumControllerImpl.kt

...oject/briar/headless/messaging/MessagingControllerImpl.kt

...project/briar/headless/messaging/OutputPrivateResponse.kt

...t/briar/headless/messaging/MessagingControllerImplTest.kt