save the images on a background thread

app/src/main/kotlin/com/simplemobiletools/gallery/activities/ViewPagerActivity.kt

@@ -232,6 +232,7 @@ class ViewPagerActivity : SimpleActivity(), ViewPager.OnPageChangeListener, View
     private fun saveImageAs() {
         val currPath = getCurrentPath()
         SaveAsDialog(this, currPath) {
+            Thread({
                 val selectedFile = File(it)
                 val tmpFile = File(selectedFile.parent, "tmp_${it.getFilenameFromPath()}")
                 try {
@@ -253,6 +254,7 @@ class ViewPagerActivity : SimpleActivity(), ViewPager.OnPageChangeListener, View
                     toast(R.string.unknown_error_occurred)
                     deleteFile(tmpFile) {}
                 }
+            }).start()
         }
     }