capture-mode-polish: Ignore Fully Occluded Windows in kWindow

Currently, there is no algorithm to detect if a window is fully
occluded in kWindow mode of screen capture session. Adding window
occlusion detection algorithm to decide if a window is fully
occluded and updating the logic to decide if a window is selectable
accordingly.

Change-Id: I491ea4d23712f81132f76d2722678c4320c24a37
Bug: 1318231
Test: Manually + add unittest
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3642242
Commit-Queue: Michele Fan <michelefan@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1006715}