[Tab Collection] Implement representative tab index

This CL implements representative tab index. This works by stashing the
last time a tab is selected in a group on the TabGroupAndroid object.
In the event the last selected tab is removed from the group via a
tab closure or any call to moveTabInternal a new tab in the group
is selected. The tab that is selected follows the same logic as
picking a nearby tab to use after a tab closure, but is constrained
to the tab group.

As part of this change some of the C++ implementation is refactored and
reorganized to be a bit more convenient to use.

Testing remains limited until tab group merging is implemented which
should be the next CL.

Bug: 429145597
Change-Id: Ia2096c60bc72073e1b9f77e1516653046e2c1295
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6764850
Reviewed-by: Sky Malice <skym@chromium.org>
Auto-Submit: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1488895}