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* Introduce assistant_state_proxy as a convenient mojom proxy client for
  voice_interaction_controller.
* Introduce assistant_state_base as POD that can be shared between
  assistant_state_proxy and voice_interaction_controller to store state
* Change assistant state accessers to base::Optional to reflect the async
  nature of reading these values.
* Remove async Is___Enabled(Callback). The observer now will initialize
  state after attach.
* Fixes assistant_manager_service_impl start up sequence to avoid potential
  multiple restarts with async assistant pref queries
* Fixes a potential race when restarting assistant_manager

Bug: b/112281490
Test: locally build and run
Change-Id: I7c57adef0919583283f2fb1312f6d6e191bf266a
Reviewed-on: https://chromium-review.googlesource.com/c/1303333
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604678}