Add Constructed/Destroying events to AssistantControllerObserver.

These events are used to reduce the complexity of the circular
dependency between AssistantController's sub-controllers.

Bug: b:80542452
Change-Id: If266ca01f844ec97d7f0b5f6a7216dac0da4e17f
Reviewed-on: https://chromium-review.googlesource.com/1133587
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574631}
9 files changed