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This simplifies the code in multiple ways:
   1) BloomControllerImpl no longer relies on any of the other Assistant
      classes, like |AssistantInteractionObserver| and
      |AssistantActionObserver|.
   2) BloomControllerImpl no longer needs to expose methods like
      HasInteraction() and GetLastInteractionResolution() just for the
      purpose of unittests.

Bug: b/165356952
Tests: chromeos_components_unittests --gtest_filter="Bloom*.*"
Change-Id: I150c2be11ccad611bf468ad33c5835e9c7146d7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2365753
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802777}