Add notification channel support.

Bucket our notifications into channels. Required to target API 26.

src/org/thoughtcrime/securesms/notifications/AbstractNotificationBuilder.java

@@ -31,6 +31,7 @@ public abstract class AbstractNotificationBuilder extends NotificationCompat.Bui
     this.context = context;
     this.privacy = privacy;
 
+    setChannelId(NotificationChannels.MESSAGES);
     setLed();
   }