Pulled out AssistantScreenContextController from AssistantController.

In addition, this CL:
- Adds an AssistantScreenContextModel/Observer.
- Inactivates the Assistant widget on show.
- Activates the Assistant widget on screen context request finished.

The AssistantScreenContextModel will also be used to cache contextual
cards returned from the server. This will be added in a follow up CL.

Bug: b:111008970
Change-Id: I0cd2cb782b880abc520bc4a8fca3c82acc1575c7
Reviewed-on: https://chromium-review.googlesource.com/1131914
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Muyuan Li <muyuanli@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574341}
14 files changed