change sync interval on change in code

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/service/KeyserverSyncAdapterService.java

@@ -11,6 +11,7 @@ import android.content.ContentProviderClient;
 import android.content.ContentResolver;
 import android.content.Context;
 import android.content.Intent;
+import android.content.PeriodicSync;
 import android.content.SyncResult;
 import android.database.Cursor;
 import android.graphics.Bitmap;
@@ -529,6 +530,10 @@ public class KeyserverSyncAdapterService extends Service {
         return builder.build();
     }
 
+    /**
+     * creates a new sync if one does not exist, or updates an existing sync if the sync interval
+     * has changed.
+     */
     public static void enableKeyserverSync(Context context) {
         Account account = KeychainApplication.createAccountIfNecessary(context);
 
@@ -539,12 +544,26 @@ public class KeyserverSyncAdapterService extends Service {
 
         ContentResolver.setIsSyncable(account, Constants.PROVIDER_AUTHORITY, 1);
         ContentResolver.setSyncAutomatically(account, Constants.PROVIDER_AUTHORITY, true);
-        ContentResolver.addPeriodicSync(
-                account,
-                Constants.PROVIDER_AUTHORITY,
-                new Bundle(),
-                SYNC_INTERVAL
-        );
+
+        boolean intervalChanged = false;
+        boolean syncExists = Preferences.getKeyserverSyncEnabled(context);
+
+        if (syncExists) {
+            long oldInterval = ContentResolver.getPeriodicSyncs(
+                    account, Constants.PROVIDER_AUTHORITY).get(0).period;
+            if (oldInterval != SYNC_INTERVAL) {
+                intervalChanged = true;
+            }
+        }
+
+        if (!syncExists || intervalChanged) {
+            ContentResolver.addPeriodicSync(
+                    account,
+                    Constants.PROVIDER_AUTHORITY,
+                    new Bundle(),
+                    SYNC_INTERVAL
+            );
+        }
     }
 
     private boolean isSyncEnabled() {