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This is a refactor that simplifies using voice interaction observer

Bug: 870515
Test: locally compile and run
Change-Id: I60368068c1124e9f8d0a6901fff23c1eb5b05717
Reviewed-on: https://chromium-review.googlesource.com/c/1278862Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599345}