Session: Add a shortcut in detecting if a window that contains the wallpaper is null or not

originally, a null window would be detected in WindowManagerService, an exception would be thrown, and then that would be caught in Session

Change-Id: Ib937d029bb7a27dac15ff38ecf3b4fbb51bc417c
NotificationManagerService: notifications with icon==0 are FINE WITH US

This warning reminds me of that saying about squeaky wheels... "the squeaky wheel gets PUNCHED IN THE HEAD"

Change-Id: I3e1a1e55ce3ce1f70c7b187a237625257bdbd591
Signed-off-by: pcarenza <pcarenza@gmail.com>