Limit number of tasks when limiting query threads

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

@@ -289,6 +289,10 @@ public class ApplicationEx extends Application {
                 editor.putString("subject_ellipsize", "middle");
             if (!prefs.contains("auto_optimize"))
                 editor.putBoolean("auto_optimize", false);
+        } else if (version < 1253) {
+            int threads = prefs.getInt("query_threads", 4);
+            if (threads == 4)
+                editor.remove("query_threads");
         }
 
         if (version < BuildConfig.VERSION_CODE)

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

@@ -264,7 +264,10 @@ public class FragmentOptionsMisc extends FragmentBase implements SharedPreferenc
         swQueries.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
             @Override
             public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
-                prefs.edit().putInt("query_threads", checked ? 2 : 4).commit(); // apply won't work here
+                if (checked)
+                    prefs.edit().putInt("query_threads", 2).commit(); // apply won't work here
+                else
+                    prefs.edit().remove("query_threads").commit(); // apply won't work here
                 restart();
             }
         });

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

@@ -21,6 +21,7 @@ package eu.faircode.email;
 
 import android.content.Context;
 import android.content.Intent;
+import android.content.SharedPreferences;
 import android.os.Bundle;
 import android.os.Handler;
 
@@ -33,6 +34,7 @@ import androidx.lifecycle.LifecycleOwner;
 import androidx.lifecycle.LifecycleService;
 import androidx.lifecycle.OnLifecycleEvent;
 import androidx.localbroadcastmanager.content.LocalBroadcastManager;
+import androidx.preference.PreferenceManager;
 
 import java.util.ArrayList;
 import java.util.Date;
@@ -54,11 +56,9 @@ public abstract class SimpleTask<T> implements LifecycleObserver {
     private String name;
     private Future<?> future;
 
+    private static ExecutorService executor = null;
     private static final List<SimpleTask> tasks = new ArrayList<>();
 
-    private static final ExecutorService executor =
-            Helper.getBackgroundExecutor(Runtime.getRuntime().availableProcessors(), "task");
-
     static final String ACTION_TASK_COUNT = BuildConfig.APPLICATION_ID + ".ACTION_TASK_COUNT";
 
     public SimpleTask<T> setLog(boolean log) {
@@ -117,6 +117,13 @@ public abstract class SimpleTask<T> implements LifecycleObserver {
             onException(args, ex);
         }
 
+        if (executor == null) {
+            SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
+            int threads = prefs.getInt("query_threads", Runtime.getRuntime().availableProcessors());
+            Log.i("Task threads=" + threads);
+            executor = Helper.getBackgroundExecutor(threads, "task");
+        }
+
         future = executor.submit(new Runnable() {
             private Object data;
             private long elapse;