[Select-to-speak] Fix mouse hovers passing through Select-to-speak

After activating Select-to-speak, mouse hovers should be suppressed from
passing through to UI elements underneath the mouse cursor.

Also tested with sticky keys (e.g. pressing search, and dragging)
(to prevent regressing crbug.com/819860)

This also moves cursor, magnifier, and autoclick event handling to a
higher event processing level.

Fixed: 1372639, 1373211, 1373217
AX-Relnotes: Select-to-speak now correctly suppresses hovers on underlying elements when activated.
Change-Id: I12a19e0cc8bd748e78118164d36b31b526ace523
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3945689
Reviewed-by: Katie Dektar <katie@chromium.org>
Commit-Queue: Josiah Krutz <josiahk@google.com>
Cr-Commit-Position: refs/heads/main@{#1062410}