Assistant: Keep all notifications

This patch keeps all the assistant notifications in message center.

Bug: b/79493568
Test: manual.
Change-Id: If877634199fb1a0a7a269fe273d0cf6f3283cc15
Reviewed-on: https://chromium-review.googlesource.com/1137487
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575498}
2 files changed