[Eche] Add new views for recent apps loading state

This CL introduces new views for the recent apps view to represent a
loading state. A follow-up CL will include the logic for displaying the
loading view, error view, and transition to the normal recent apps
buttons view.

Screen capture showing the animation:
https://drive.google.com/file/d/1uEOnaXQUT7SY2aug-e663vu-yUeqPgsR/view?usp=sharing&resourcekey=0-bxDJXQsjU0IS7W3B61cqbg

Test: Manually verified that when setting loading_view_ to visible it
will show. Verified that the non-loading view and buttons still work
as intended.

Bug: b/271478560
Change-Id: Ifb1d1cb8830dadbd5164a774a9eaa116cb024754
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4307837


Reviewed-by: Jon Mann <jonmann@chromium.org>
Commit-Queue: Curt Clemens <cclem@google.com>
Reviewed-by: Pu Shi <pushi@google.com>
Cr-Commit-Position: refs/heads/main@{#1118447}