Fixed that notifications could be stuck and invisible on AOD

Notifications could get stuck if they were pulsing while the
prox sensor was covered. We're now only showing notifications
if we're pulsing.

This also fixes a bug around the background not drawing properly
while pulsing.

Fixes: 129552328
Test: atest SystemUITests
Change-Id: I499c6c167b155c69a2f8313850ca467da9ea295a