[headless] expose ContactManager methods for adding contacts remotely

briar-headless/README.md

briar-headless/build.gradle

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

.../briarproject/briar/headless/contact/ContactController.kt

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

...iarproject/briar/headless/contact/OutputPendingContact.kt

...a/org/briarproject/briar/headless/BriarHeadlessTestApp.kt

...t/java/org/briarproject/briar/headless/IntegrationTest.kt

...riar/headless/contact/ContactControllerIntegrationTest.kt

...arproject/briar/headless/contact/ContactControllerTest.kt