[TSLO] Repurpose WindowInsetsUtils#getWidestUnoccludedRect() to
additionally return if the unoccluded region is complex

Also update AppHeaderCoordinator heuristics checks to avoid customizing
the app header when the unoccluded region in the caption bar is complex.

OBSOLETE_HISTOGRAM[Android.DesktopWindowHeuristicResult3]=Replaced by Android.DesktopWindowHeuristicResult4

Bug: 418868898
Change-Id: I8cf3dc1ef5e466ed4464ffc83319360deeff3652
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6574741
Reviewed-by: Wenyu Fu <wenyufu@chromium.org>
Commit-Queue: Aishwarya Rajesh <aishwaryarj@google.com>
Cr-Commit-Position: refs/heads/main@{#1467189}