[TabletTabGroup] Add dimming/undimming animation for hovered groups

Add dimming/undimming animation for hovered groups.

When in reorder mode, if the dragged tab enters the threshold of a tab
group, that tab group will be undimmed (and vice versa).

Video: https://drive.google.com/file/d/1mHqS5mjeaYNyLvAyBN95dUxbJrOo-1DN/view?usp=sharing

Bug: 1348332
Change-Id: Ibecc866940811ec0e884bf53ce69944202a200be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3863992


Reviewed-by: Sirisha Kavuluru <skavuluru@google.com>
Commit-Queue: Neil Coronado <nemco@google.com>
Cr-Commit-Position: refs/heads/main@{#1041699}