[Tabs] Update IncognitoTabGroupModelFilterImpl structure

Restructure how IncognitoTabGroupModelFilterImpl sets its internal
filter by observing the IncognitoTabModelImpl. This avoids a circular
initialization dependency that otherwise made the class very hard to
set up without awkward suppliers.

Bug: 423043938
Change-Id: I48ed5500d4a85928256c90491d75779646b83860
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6637891
Auto-Submit: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1472609}