assistant: update owners

Bug: None
Test: None
Change-Id: Ie6f0e98b4e1a1d3bd1756734f38c71c1a2814fa0
Reviewed-on: https://chromium-review.googlesource.com/1217853Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590400}

chromeos/assistant/OWNERS

-xiaohuic@chromium.org
 muyuanli@chromium.org
+updowndota@chromium.org
+wutao@chromium.org
+xiaohuic@chromium.org
 
 # COMPONENTS: UI>Shell>Assistant