bruschetta: Add a default icon.

This is what shows up in the launcher if an app doesn't supply its own
icon. Previously, it would look like the existing Crostini icon.

Bug: b/247636749
Test: manually checked Crostini and Bruschetta app groups on a DUT.
Change-Id: I86f34802cdb0714650351ea216e3ff283d25d525
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4685031


Reviewed-by: David Munro <davidmunro@google.com>
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Emil Mikulic <easy@google.com>
Cr-Commit-Position: refs/heads/main@{#1171622}