CCA: Add feature flag for CrOS Events

Bug: b:325853230
Test: Manually
Change-Id: I76c0cf315dc5043a5683e0944fdf775579123723
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5305922


Reviewed-by: Chu-Hsuan Yang <chuhsuan@chromium.org>
Commit-Queue: Wei Lee <wtlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1263890}