Simplify FedCmAccountSelectionView (part 3/N)

This CL deletes AccountSelectionViewBase::Observer and replaces
callsites with FedCmAccountSelectionView. FedCmAccountSelectionView and
AccountSelectionViewBase are already tightly coupled with multiple
layers of re-entrancy. The Observer interface adds abstraction and
complexity for no benefit. Removing it and directly referencing
FedCmAccountSelectionView makes the existing logic clearer, and will
make it easier to move code around for subsequent refactors.

Bug: 377803489
Change-Id: I40cc63c5e603c9c091140264c3175f704d16a60d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6010127
Reviewed-by: Nicolás Peña <npm@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1383364}