define strangers as people not in roster instead of no mutual presence subscription

src/main/java/eu/siacs/conversations/entities/Conversation.java

@@ -1009,7 +1009,7 @@ public class Conversation extends AbstractEntity implements Blockable, Comparabl
 	public boolean isWithStranger() {
 		return mode == MODE_SINGLE
 				&& !getJid().equals(account.getJid().toDomainJid())
-				&& !getContact().mutualPresenceSubscription()
+				&& !getContact().showInRoster()
 				&& sentMessagesCount() == 0;
 	}