[App Management] Extracting common elements in main view and notification view to a new element.

This CL extracts the common elements in notification and main view, a expandable app list, to a new element called expandable_app_list.

Bug: 906508
Change-Id: I28530b5c047f21dfa3910290d7804588d9f0360a
Reviewed-on: https://chromium-review.googlesource.com/c/1447393
Commit-Queue: Cecilia Ni <ceciliani@google.com>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632504}
11 files changed