trigger an update after joining good wifi without JobSchedule

Each time the device connects to a wifi network, this waits for 2 minutes,
then if the wifi is still connected, it re-schedules the index update to
happen now.  The goal is to favor unmetered networks as much as possible
when downloading the index and any automatic app updates.

This is only needed on older platforms, JobScheduler handles this for us on
android-21+

app/src/main/java/org/fdroid/fdroid/UpdateService.java

@@ -27,6 +27,7 @@ import android.content.Context;
 import android.content.Intent;
 import android.content.IntentFilter;
 import android.content.SharedPreferences;
+import android.os.AsyncTask;
 import android.os.Build;
 import android.os.Handler;
 import android.os.Looper;
@@ -51,6 +52,7 @@ import org.fdroid.fdroid.net.BluetoothDownloader;
 import org.fdroid.fdroid.net.ConnectivityMonitorService;
 import org.fdroid.fdroid.views.main.MainActivity;
 
+import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -147,6 +149,51 @@ public class UpdateService extends IntentService {
         return updating;
     }
 
+    private static volatile boolean isScheduleIfStillOnWifiRunning;
+
+    /**
+     * Waits for a period of time for the WiFi to settle, then if the WiFi is
+     * still active, it schedules an update.  This is to encourage the use of
+     * unlimited networks over metered networks for index updates and auto
+     * downloads of app updates. Starting with {@code android-21}, this uses
+     * {@link android.app.job.JobScheduler} instead.
+     */
+    public static void scheduleIfStillOnWifi(Context context) {
+        if (Build.VERSION.SDK_INT >= 21) {
+            throw new IllegalStateException("This should never be used on android-21 or newer!");
+        }
+        if (isScheduleIfStillOnWifiRunning || !Preferences.get().isBackgroundDownloadAllowed()) {
+            return;
+        }
+        isScheduleIfStillOnWifiRunning = true;
+        new StillOnWifiAsyncTask(context).execute();
+    }
+
+    private static final class StillOnWifiAsyncTask extends AsyncTask<Void, Void, Void> {
+
+        private final WeakReference<Context> contextWeakReference;
+
+        private StillOnWifiAsyncTask(Context context) {
+            this.contextWeakReference = new WeakReference<>(context);
+        }
+
+        @Override
+        protected Void doInBackground(Void... voids) {
+            Context context = contextWeakReference.get();
+            try {
+                Thread.sleep(120000);
+                if (Preferences.get().isBackgroundDownloadAllowed()) {
+                    Utils.debugLog(TAG, "scheduling update because there is good internet");
+                    schedule(context);
+                }
+            } catch (Exception e) {
+                Utils.debugLog(TAG, e.getMessage());
+            }
+            isScheduleIfStillOnWifiRunning = false;
+            return null;
+        }
+    }
+
     @Override
     public void onCreate() {
         super.onCreate();

app/src/main/java/org/fdroid/fdroid/net/WifiStateChangeService.java

@@ -8,6 +8,7 @@ import android.net.DhcpInfo;
 import android.net.NetworkInfo;
 import android.net.wifi.WifiInfo;
 import android.net.wifi.WifiManager;
+import android.os.Build;
 import android.support.annotation.Nullable;
 import android.support.v4.content.LocalBroadcastManager;
 import android.text.TextUtils;
@@ -15,6 +16,7 @@ import android.util.Log;
 import org.apache.commons.net.util.SubnetUtils;
 import org.fdroid.fdroid.FDroidApp;
 import org.fdroid.fdroid.Preferences;
+import org.fdroid.fdroid.UpdateService;
 import org.fdroid.fdroid.Utils;
 import org.fdroid.fdroid.data.Repo;
 import org.fdroid.fdroid.localrepo.LocalRepoKeyStore;
@@ -42,6 +44,11 @@ import java.util.Locale;
  * changed.  Having the {@code Thread} also makes it easy to kill work
  * that is in progress.
  * <p>
+ * This also schedules an update to encourage updates happening on
+ * unmetered networks like typical WiFi rather than networks that can
+ * cost money or have caps.  The logic for checking the state of the
+ * internet connection is in {@link org.fdroid.fdroid.UpdateService#onHandleIntent(Intent)}
+ * <p>
  * Some devices send multiple copies of given events, like a Moto G often
  * sends three {@code CONNECTED} events.  So they have to be debounced to
  * keep the {@link #BROADCAST} useful.
@@ -92,6 +99,10 @@ public class WifiStateChangeService extends IntentService {
                 wifiInfoThread = new WifiInfoThread();
                 wifiInfoThread.start();
             }
+
+            if (Build.VERSION.SDK_INT < 21 && wifiState == WifiManager.WIFI_STATE_ENABLED) {
+                UpdateService.scheduleIfStillOnWifi(this);
+            }
         }
     }