Limit number of notifications to 20 per group

app/src/main/java/eu/faircode/email/Core.java

@@ -112,6 +112,7 @@ import me.leolin.shortcutbadger.ShortcutBadger;
 import static android.os.Process.THREAD_PRIORITY_BACKGROUND;
 
 class Core {
+    private static final int MAX_NOTIFICATION_COUNT = 20; // per group
     private static final int SYNC_BATCH_SIZE = 20;
     private static final int DOWNLOAD_BATCH_SIZE = 20;
     private static final long YIELD_DURATION = 200L; // milliseconds
@@ -1776,7 +1777,8 @@ class Core {
                         Log.i("Notify existing=" + id);
                     } else {
                         remove.remove(-id);
-                        add.add(id);
+                        if (add.size() < MAX_NOTIFICATION_COUNT)
+                            add.add(id);
                         Log.i("Notify adding=" + id);
                     }
                 }