briar-headless: Inject a singleton ObjectMapper for JSON parsing

...in/java/org/briarproject/briar/headless/HeadlessModule.kt

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

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

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

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

...st/java/org/briarproject/briar/headless/ControllerTest.kt

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

...briarproject/briar/headless/forums/ForumControllerTest.kt

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