GitLab

This CL:
- Makes PointerDeviceObserver get notified when a mouse or touchpad is
  plugged in or removed by making PointerDeviceObserver an
  InputDeviceEventObserver on ozone only.
- Adds InputDeviceEventObserver::OnMouseDeviceConfigurationChanged() and
       InputDeviceEventObserver::OnTouchpadDeviceConfigurationChanged().
- Adds the plumbing from EventFactoryEvdev to notify
  InputDeviceEventObservers when a mouse or touchpad is plugged in or
  removed.

In a later CL I will make PointerDeviceObserver an InputDeviceEventObserver on
X11 and add the X11 plumbing

BUG=450899
TEST=Manual, see bug

Review URL: https://codereview.chromium.org/889403003

Cr-Commit-Position: refs/heads/master@{#314400}