only update static WiFi settings var from WifiInfoThread

Since Intents can come in any time, whether WifiInfoThread is running or
not, the global static vars for storing the WiFi settings info should only
be updated from the WifiInfoThread.  Otherwise, the WiFi settings could be
nulled out between the time of the null guard and the execution in code
like this:

if (!TextUtils.isEmpty(FDroidApp.ipAddressString) && netmask != null) {
  FDroidApp.subnetInfo = new SubnetUtils(FDroidApp.ipAddressString, netmask).getInfo();

fixes #589 https://gitlab.com/fdroid/fdroidclient/issues/589

java.lang.RuntimeException: An error occured while executing doInBackground()
        at android.os.AsyncTask$3.done(AsyncTask.java:304)
        at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
        at java.util.concurrent.FutureTask.run(FutureTask.java:242)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:818)
Caused by: java.lang.IllegalArgumentException: Could not parse [null/24]
        at org.apache.commons.net.util.SubnetUtils.calculate(SubnetUtils.java:275)
        at org.apache.commons.net.util.SubnetUtils.<init>(SubnetUtils.java:62)
        at org.fdroid.fdroid.net.WifiStateChangeService$WaitForWifiAsyncTask.doInBackground(WifiStateChangeService.java:89)
        at org.fdroid.fdroid.net.WifiStateChangeService$WaitForWifiAsyncTask.doInBackground(WifiStateChangeService.java:70)
        at android.os.AsyncTask$2.call(AsyncTask.java:292)
        at java.util.concurrent.FutureTask.run(FutureTask.java:237)
        ... 4 more
java.lang.IllegalArgumentException: Could not parse [null/24]
        at org.apache.commons.net.util.SubnetUtils.calculate(SubnetUtils.java:275)
        at org.apache.commons.net.util.SubnetUtils.<init>(SubnetUtils.java:62)
        at org.fdroid.fdroid.net.WifiStateChangeService$WaitForWifiAsyncTask.doInBackground(WifiStateChangeService.java:89)
        at org.fdroid.fdroid.net.WifiStateChangeService$WaitForWifiAsyncTask.doInBackground(WifiStateChangeService.java:70)
        at android.os.AsyncTask$2.call(AsyncTask.java:292)
        at java.util.concurrent.FutureTask.run(FutureTask.java:237)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:818)

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

@@ -139,7 +139,9 @@ public class FDroidApp extends Application {
     }
 
     /**
-     * Initialize the settings needed to run a local swap repo.
+     * Initialize the settings needed to run a local swap repo. This should
+     * only ever be called in {@link org.fdroid.fdroid.net.WifiStateChangeService.WifiInfoThread},
+     * after the single init call in {@link FDroidApp#onCreate()}.
      */
     public static void initWifiSettings() {
         port = 8888;

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

@@ -53,7 +53,6 @@ public class WifiStateChangeService extends IntentService {
     protected void onHandleIntent(Intent intent) {
         android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_LOWEST);
         Utils.debugLog(TAG, "WiFi change service started, clearing info about wifi state until we have figured it out again.");
-        FDroidApp.initWifiSettings();
         NetworkInfo ni = intent.getParcelableExtra(WifiManager.EXTRA_NETWORK_INFO);
         wifiManager = (WifiManager) getSystemService(WIFI_SERVICE);
         int wifiState = wifiManager.getWifiState();
@@ -79,6 +78,7 @@ public class WifiStateChangeService extends IntentService {
         public void run() {
             android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_LOWEST);
             try {
+                FDroidApp.initWifiSettings();
                 Utils.debugLog(TAG, "Checking wifi state (in background thread).");
                 WifiInfo wifiInfo = null;