[Mahi] refactor: Introduce MahiUiController::State

This CL introduces MahiUiController::State, corresponding to three
UI states that Mahi views can stay:

1.Summary & outlines state: The views that serve summary & outlines
  should show.

2.Q&A state: The views that serve Q&A should show.

3.Error state: The view that visualizes the current error should
  show.

In the subsequent CL, MahiUiController is gonna cache the current
state. Because we will have UIs that rely on the current state,
such as the retry link. If the current state is summary & outlines,
clicking the retry link should request summary & outlines again;
if the current state is Q&A, clicking the retry link should request
the answer to the most recent question.

This CL also moves base::ScopedObservation inside
MahiUiController::Observer so that the observer subclasses do not
have to create their own observations.

This CL should not bring any UI difference.

Bug: b/319731862
Change-Id: Iabc2f07b0dc0f0385b0169e49fc10a8e3800bcb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407613
Reviewed-by: Andre Le <leandre@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280420}