Import: replaced async task with thread.

Extracted directory chooser into a separate activity.

src/main/java/de/dennisguse/opentracks/util/FileUtils.java

@@ -29,6 +29,7 @@ import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.nio.channels.FileChannel;
 import java.text.SimpleDateFormat;
+import java.util.ArrayList;
 import java.util.Date;
 
 import de.dennisguse.opentracks.BuildConfig;
@@ -111,6 +112,10 @@ public class FileUtils {
         return fileName.substring(index + 1);
     }
 
+    public static String getExtension(DocumentFile file) {
+        return getExtension(file.getName());
+    }
+
     /**
      * Builds a filename with the given base and the given extension, possibly adding a suffix to ensure the file doesn't exist.
      *
@@ -229,13 +234,13 @@ public class FileUtils {
         return file.getAbsolutePath();
     }
 
-        /**
+    /**
      * Copy a File (src) to a File (dst).
      *
      * @param src source file.
      * @param dst destination file.
-         */
-        public static void copy(FileDescriptor src, File dst) {
+     */
+    public static void copy(FileDescriptor src, File dst) {
         try (FileChannel in = new FileInputStream(src).getChannel();
              FileChannel out = new FileOutputStream(dst).getChannel()) {
             in.transferTo(0, in.size(), out);
@@ -262,7 +267,7 @@ public class FileUtils {
      * @param context the Context.
      * @param trackId the id of the Track.
      * @param uri     the uri to check.
-     * @return        File object or null.
+     * @return File object or null.
      */
     public static File getPhotoFileIfExists(Context context, long trackId, Uri uri) {
         if (uri == null) {
@@ -300,4 +305,18 @@ public class FileUtils {
             file.delete();
         }
     }
+
+    public static ArrayList<DocumentFile> getFiles(DocumentFile file) {
+        ArrayList<DocumentFile> files = new ArrayList<>();
+
+        for (DocumentFile candidate : file.listFiles()) {
+            if (!candidate.isDirectory()) {
+                files.add(candidate);
+            } else {
+                files.addAll(getFiles(candidate));
+            }
+        }
+
+        return files;
+    }
 }