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Leaves HoldingSpaceTrayIcon to only handle the content forward icon.
Adds another image view for the default icon, and lets HoldingSpaceTray
manage which icon is visible.

Removes holding space observation logic from HoldingSpaceTrayIcon to
avoid duplicate event handling to avoid HoldingSpaceTrayIcon getting
duplicate holding space model events - e.g. if the icon is created
or initialized during OnHoldingSpaceModelAttached, and starts observing
model changes itself, it may handle model attachment twice. Relying on
HoldingSpaceTray to notify HoldingSpaceTrayIcon about model changes
makes avoid such scenario easier.

The end goal is to make HoldingSpaceTrayIcon not depend on individual
holding space model changes, and just have an API that would specify the
list of items whose previews should be shown in the tray icon - will be
done in a follow-up.

BUG=1142572

Change-Id: Iad3bc7089d2aec770e03ef88bf168d3e43f706da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586554
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#836273}