system-web-app: fix web app shortcut missing icons in sample SWA and tests

Sample SWA and test SWA didn't provide web app shortcuts menu item
icons.

This CL fixes relevant code and adds icons to WebAppInstallInfo, updates
SystemWebAppManagerInstallAllAppsBrowserTest.BasicConsistencyCheck to
verify shortcut menu item icon is provided for each shortcut entry.

Bug: 1427444,1417955
Change-Id: Ib08880900ea0f23e238602756af263a3749f99f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4613592


Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Commit-Queue: Dibyajyoti Pal <dibyapal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1158130}