Mms: Mark messages read if they are on-screen when phone is unlocked

Previous behavior: Open Messaging app, open one of the existing conversations, 
and then lock the phone. Receive a new text message in that conversation. 
Unlock the phone to reveal the new message, but the notification does not 
disappear despite the new message having been read.

This is due to onResume() not getting called read when returning to
the app from the lockscreen, so the new message never gets marked as read!

The solution is to use onWindowFocusChanged() as a failsafe for this scenario, 
as this DOES get called when you come in from the lockscreen. Now the notification 
disappears correctly. 

Change-Id: I225a31bea9abe553c1466e070f4c9b50245f05c5