capture-game: Show camera preview widget for game capture session

This CL makes the camera preview widget show inside the game
window if camera is available and selected.

Demo: http://b/283810920#comment2
Fixed: b/283810920
Test: Added unit test + manual
Change-Id: I2cddc5c9dac5363bf8957ae53a20f1cb0b25c64a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4554377


Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Michele Fan <michelefan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148677}