[App Management] Refactor main view

This CL adds an app-management-item element, and creates shared constants,
css and vars files.

Bug: 906508
Change-Id: I29dce3a6745e8957ee1fb547cbe2e72290567f28
Reviewed-on: https://chromium-review.googlesource.com/c/1371308
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@{#616156}
10 files changed