[SharedTabGroups] Add share icon to SavedTabGroupButtons

SavedTabGroupButtons now display a people group icon if they are shared.

The share icon is displayed before the title of the group.

If the group has no title we will only display the share icon. The
squircle icon will only be displayed for no title un-shared groups.

Pixel tests will be added in a follow up CL. This is because the
observers don't fire correctly when a group is shared at this point in time.

Screenshots of changes added to the attached bug.

Change-Id: Ib2715c22a0258de3f11a9a4b5b967ada5383ab03
Bug: 372741375
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5926152
Commit-Queue: Darryl James <dljames@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: David Pennington <dpenning@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378450}