add overload for exporter that takes single track

src/main/java/de/dennisguse/opentracks/fragments/ExportProgressDialogFragment.java

@@ -218,7 +218,7 @@ public class ExportProgressDialogFragment extends DialogFragment {
             }
 
             try (OutputStream outputStream = context.getContentResolver().openOutputStream(file.getUri())) {
-                if (trackExporter.writeTrack(new Track[]{track}, outputStream)) {
+                if (trackExporter.writeTrack(track, outputStream)) {
                     return true;
                 } else {
                     if (!file.delete()) {

src/main/java/de/dennisguse/opentracks/io/file/exporter/FileTrackExporter.java

@@ -54,6 +54,11 @@ public class FileTrackExporter implements TrackExporter {
         this.trackWriter = trackWriter;
     }
 
+    @Override
+    public boolean writeTrack(Track track, @NonNull OutputStream outputStream) {
+        return writeTrack(new Track[]{track}, outputStream);
+    }
+
     @Override
     public boolean writeTrack(Track[] tracks, @NonNull OutputStream outputStream) {
         try {

src/main/java/de/dennisguse/opentracks/io/file/exporter/KmzTrackExporter.java

@@ -69,6 +69,11 @@ public class KmzTrackExporter implements TrackExporter {
         this.exportPhotos = exportPhotos;
     }
 
+    @Override
+    public boolean writeTrack(Track track, @NonNull OutputStream outputStream) {
+        return writeTrack(new Track[]{track}, outputStream);
+    }
+
     @Override
     public boolean writeTrack(Track[] tracks, @NonNull OutputStream outputStream) {
         ZipOutputStream zipOutputStream = null;

src/main/java/de/dennisguse/opentracks/io/file/exporter/TrackExporter.java

@@ -29,12 +29,7 @@ import de.dennisguse.opentracks.content.data.Track;
  */
 public interface TrackExporter {
 
-    /**
-     * Write track to an output stream.
-     * Depending on the implementation a context might be required.
-     *
-     * @param tracks
-     * @param outputStream the output stream
-     */
     boolean writeTrack(Track[] tracks, @NonNull OutputStream outputStream);
+
+    boolean writeTrack(Track track, @NonNull OutputStream outputStream);
 }