app_list: add xiyuan@ as an owner of //ash/shell/*app_list*

Bug: 733662
Change-Id: Ida6406a14b3a92ad2fa85fd4c376344f64989555
Reviewed-on: https://chromium-review.googlesource.com/877041Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jiaquan He <hejq@google.com>
Cr-Commit-Position: refs/heads/master@{#530694}

ash/shell/OWNERS

+per-file *app_list*=xiyuan@chromium.org