assistant: add app_list/views/assistant/OWNERS

Bug: None
Test: None
Change-Id: I8e227f866625d1d9c85ad4ea6b8d000d5a1360a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937515Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719332}

ash/app_list/views/assistant/OWNERS

+file://chromeos/assistant/OWNERS
+
+# COMPONENT: UI>Shell>Assistant