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Conversation starters are suggestions that show when Assistant UI is
launched. They transcend Assistant sessions in that they are pre-cached
and held in memory until needed.

To accomplish this, this CL:
- Adds AssistantCacheController/AssistantCacheModel(Observer)
- Adds support for conversation starters to SuggestionContainerView.

Still TODO:
- Limit to four conversation starters.
- Conversation starters should be centered horizontally.
- Conversation starters should be cached from the server.

In the future, we can also use this new cache for engagement cards
and other growth initiatives.

See bug for mock/demo.

Bug: b:111694337
Change-Id: Ic280d7e39a5b61c9400448f883fce147a889d908
Reviewed-on: https://chromium-review.googlesource.com/1179365Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#584605}