Alert ProxCheck listeners directly when timing out.

Before this change, we were relying on the ProximitySensor class
to circle back and tell the ProximityCheck class about null results.
When the ProximitySensor class changed, this broke ProxCheck. The
ProxCheck class now directly alerts its listeners when it times out.

A test has been added to cover this case now.

Fixes: 162205584
Test: atest SystemUiTests && Manual
Change-Id: Ie48589a54daae3e29defde9ed2300f3c7e2b88ae