Include last private chat activity in list of contacts

Contact items now look like this:

{
    "author": {
        "formatVersion": 1,
        "id": "y1wkIzAimAbYoCGgWxkWlr6vnq1F8t1QRA/UMPgI0E0=",
        "name": "Test",
        "publicKey": "BDu6h1S02bF4W6rgoZfZ6BMjTj/9S9hNN7EQoV05qUo="
    },
    "contactId": 1,
    "alias" : "A local nickname",
    "handshakePublicKey": "XnYRd7a7E4CTqgAvh4hCxh/YZ0EPscxknB9ZcEOpSzY=",
    "verified": true,
    "lastChatActivity": 1557838312175
}

Fixes #1722 (closed).

briar-headless/src/main/java/org/briarproject/briar/headless/contact/ContactControllerImpl.kt

@@ -17,6 +17,7 @@ import org.briarproject.bramble.api.event.Event
 import org.briarproject.bramble.api.event.EventListener
 import org.briarproject.bramble.api.identity.AuthorConstants.MAX_AUTHOR_NAME_LENGTH
 import org.briarproject.bramble.util.StringUtils.toUtf8
+import org.briarproject.briar.api.conversation.ConversationManager
 import org.briarproject.briar.headless.event.WebSocketController
 import org.briarproject.briar.headless.getContactIdFromPathParam
 import org.briarproject.briar.headless.getFromJson
@@ -38,6 +39,7 @@ internal class ContactControllerImpl
 @Inject
 constructor(
     private val contactManager: ContactManager,
+    private val conversationManager: ConversationManager,
     private val objectMapper: ObjectMapper,
     private val webSocket: WebSocketController
 ) : ContactController, EventListener {
@@ -61,7 +63,8 @@ constructor(
 
     override fun list(ctx: Context): Context {
         val contacts = contactManager.contacts.map { contact ->
-            contact.output()
+            val latestMsgTime = conversationManager.getGroupCount(contact.id).latestMsgTime
+            contact.output(latestMsgTime)
         }
         return ctx.json(contacts)
     }