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We need to disable it:
- when Assistant UI is dismissed.
- when an Assistant interaction is started (e.g. via hotword).

Bug: b:110961300
Change-Id: I97024e9f2681d171c40f54ac5b47c5ed549085b6
Reviewed-on: https://chromium-review.googlesource.com/1119530
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573520}