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Previously, Assistant response text/cards/suggestions were streamed in
and rendered immediately. We actually want to cache them until all
renderable elements for an interaction are received, and then show them
all at once.

This is a prerequisite for motion specs which require animating the
response in as a whole and resizing the container smoothly.

To accomplish this, this CL adds:
- AssistantResponse object to hold the renderable elements.
- A cached AssistantResponse in AssistantInteractionController that is
  not added to the model until it is finalized.
- Notification of TTS started event to AssistantInteractionController.
  This is used as an early indication that we have received all
  renderable elements for an interaction.

Bug: b:111932420
Change-Id: I0cc84079183fc30b8c949491e5e2bc4d0b4365b1
Reviewed-on: https://chromium-review.googlesource.com/1153626Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#579859}