Normalize export to fle and share link behaviour.

Summary:
Both export methods behaved slightly differently:
* Export to file returned earlier when running in background, which skiped the tracking of export termination;
* Share link always showed notification, even when on foreground;
* Share link status "hanged" - the share was never unset.

This change makes both consistent:
* Always track export finish - whether its in foreground or background;
* Only show notificaiton if running in background;
* Always reset the share/status.

On top of this it also:
* Normalizes the screenshot status to terminate in '...' as all the others;
* Only copies export URL to clipboard when exporting link if running in the background.

Reviewed By: passy

Differential Revision: D21425095

fbshipit-source-id: 9864a63269df6bd05ab065ff0e5d9f17b9ac6db6

desktop/app/src/chrome/ShareSheetExportFile.tsx

@@ -125,15 +125,14 @@ export default class ShareSheetExportFile extends Component<Props, State> {
         ),
         `${EXPORT_FLIPPER_TRACE_EVENT}:UI_FILE`,
       );
-      this.store.dispatch(unsetShare());
       if (this.state.runInBackground) {
         new Notification('Shareable Flipper Export created', {
           body: `Saved to ${this.props.file}`,
           requireInteraction: true,
         });
-        return;
       }
       this.setState({fetchMetaDataErrors, result: {kind: 'success'}});
+      this.store.dispatch(unsetShare());
       this.props.logger.trackTimeSince(mark, 'export:file-success');
     } catch (err) {
       const result: {