Remove jiameng@ from OWNERS because I've left the team

Also add thanhdng@ as an owner.

Change-Id: Idb829e0186979b9d73581dc4dff7c8c42ef0b6c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612784Reviewed-by: Thanh Nguyen <thanhdng@chromium.org>
Commit-Queue: Jia Meng <jiameng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840810}

chrome/browser/chromeos/power/auto_screen_brightness/OWNERS

 amoylan@chromium.org
-jiameng@chromium.org
 martis@chromium.org
+thanhdng@chromium.org
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