Create GameModeController in non-Borealis code path

Make GameModeController not owned by BorealisContext, so that it can
more naturally start to support ARC game mode.

This causes the GameModeController to be run in more configurations than
before, including when Borealis is not enabled. This includes test
cases, which necessitates a fix in the focus change callback.

Bug: b:228405479
Test: GameModeControllerTest
Test: Run Steam game; verify Game Mode signal maintained for 2 minutes

Change-Id: I7453efd51e8bbee76c8310210403a5f829e32ca0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3784567
Reviewed-by: Chloe Pelling <cpelling@google.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Matthew DeVore <matvore@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1033766}