Use Client Layer Events in ContactListFragment

This prevents trying to access the same group metadata in different groups.
Also, the conversation does not need to be reloaded once introduction messages arrive.

Closes #535

briar-api/src/org/briarproject/api/sharing/SharingMessage.java

@@ -22,7 +22,7 @@ public interface SharingMessage {
 		private final GroupId groupId;
 		private final SessionId sessionId;
 
-		public BaseMessage(GroupId groupId, SessionId sessionId) {
+		BaseMessage(GroupId groupId, SessionId sessionId) {
 
 			this.groupId = groupId;
 			this.sessionId = sessionId;