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This patch add the AddObserver method to voice interaction controller
mojom.

Bug: b/110094434, b/80431063
Test: manual.
Change-Id: I15853b1cea5cc11c3228f59f80df2bd634616c60
Reviewed-on: https://chromium-review.googlesource.com/1111645Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569677}