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AssistantBubbleView should be concerned with choreography of its child
views, not their specific implementations.

Will also make it easier to unit test view classes in isolation.

Note that this CL also creates a constants file for Assistant UI.

Bug: b:80245607
Change-Id: I05be99e93383c5df61c279a54b811a03a8a3cb29
Reviewed-on: https://chromium-review.googlesource.com/1072147Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#562570}