a11y: Add countdown when hovering on exit

Right now, the scroll panel hides as soon as the user hovers over the
exit button, which is not ideal because it can close by accident.

This CL fixes it by adding a delay before hiding the panel when the exit
button is hovered. The delay time is the same as the set countdown.

Video: http://shortn/_JxFXNsIXKe

Bug: b/401509893
Test: AutoclickScrollPaneTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: I537131d57e97ef83305fdb492b93423dd36596ce