app_list: Update Assistant icon when ready

Assistant will take time to start and be ready when enabled in settings.
This patch adds a callback when Assistant is ready.

Bug: b/132291789
Test: manual
Change-Id: I6fb79098d91f2ca592d88f24679d3641d3610d4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610311
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660509}