Fixed Android 7.0- compatibility

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

@@ -2837,14 +2837,18 @@ public class Helper {
     }
 
     static void secureDelete(File file) {
-        if (file.exists()) {
         try {
+            if (file.exists()) {
+                if (Build.VERSION.SDK_INT < Build.VERSION_CODES.O) {
+                    if (!file.delete())
+                        throw new FileNotFoundException(file.getAbsolutePath());
+                } else
                     Files.delete(Paths.get(file.getAbsolutePath()));
+            }
         } catch (IOException ex) {
             Log.e(ex);
         }
     }
-    }
 
     static long getAvailableStorageSpace() {
         StatFs stats = new StatFs(Environment.getDataDirectory().getAbsolutePath());