[iOS][TabStrip] Fix tab strip group cell appearance

This CL fixes a few visual issues with groups in tab strip:
- When a group title is empty, the group cell should be a circle.
- The group stroke in a group cell needs to have a minimum width.
- The group stroke color should update when switching between
  light/dark mode.
- TabStripGroupCell needs to return UIDragPreviewParameters.
- TabStripGroupCell title color needs to be white in light mode and
  black in dark mode.

Switching from light to dark mode before change:
- Light: https://screenshot.googleplex.com/4Fv2YB9yYanpvD3
- Dark: https://screenshot.googleplex.com/f4nBNc4aWzxSiv4
Switching from light to dark mode after change:
- Light: https://screenshot.googleplex.com/B3sNgGc8VbaiUt3
- Dark: https://screenshot.googleplex.com/8UMvce9u5KdNjJz

Bug: 332527271
Change-Id: I37f10dc27520bb39fbc9aec2c44f905685681b69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5412976
Reviewed-by: Ewann Pellé <ewannpv@chromium.org>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Cr-Commit-Position: refs/heads/main@{#1281805}