Stop timer when notification is not visible

This combines ag/8888937 and ag/9019277 into one CL, plus fixes another issue
where the timer could restart if the user had removed a notification and then
restarted the stream

Fixes: 138261464
Test: manual, atest NotificationMediaTemplateViewWrapperTest

Also checked the changes on the master branch since that had very
obvious issues when locking / unlocking the phone before

Change-Id: I6a0bbc675d33a5c7d4ce7f0884aec19606bff4fa