[App Management] Load icons from chrome://app-icon.

This CL changes the app management page to load its app icons from
chrome://app-icon instead of chrome://extension-icon. It also fixes
a bug which prevents the default icon from loading in some cases where
the icon can't be found.

The chrome://app-icon url currently doesn't work for some Chrome
system apps (e.g. Web Store, Audio Player), so this will require
further investigation.

Bug: 906508
Change-Id: Ib350bf2490193fd17c807d2677f05355eefeb6bf
Reviewed-on: https://chromium-review.googlesource.com/c/1428620
Commit-Queue: Reka Norman <rekanorman@google.com>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Nigel Tao <nigeltao@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626907}
3 files changed