Always change `[UIApplication sharedApplication] idleTimerDisabled]` on the main thread.
rdar://104266428
Pull request: https://github.com/WebKit/WebKit/pull/9893
Committed 260098@main (1c8de44efc1a): <https://commits.webkit.org/260098@main> Reviewed commits have been landed. Closing PR #9893 and removing active labels.