[Tab Switcher] Fix SharedImageTiles when joining in GTS

There is a window of time between when a tab group is starting to open
and finishing in LocalTabGroupMutationHelper where it is unmapped
meaning its SavedTabGroup is not findable via LocalTabGroupId.

If a SharedGroupObserver is created in this time window it will mark
the group as NOT_SHARED. However, none of the events it listened to
prior to this CL would fire after the mapping was created leading
to the observer thinking the group was perpetually unshared until any
other event occurred.

Observing onTabGroupLocalIdChanged in addition to the existing
observer methods seems to fix this because it is fired after the
group mapping has been created.

Fixed: 401328873
Change-Id: Icf2dabd50d29a09c221c324656eda12b5c9670d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6333006
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1429517}