UnifiedEmail: Set undo delete notification to PRIORITY_MIN

It bothers my OCD that after I press delete, the undo notification still causes a little
unread Email icon in the status bar. This takes care of that. :-P

Conflicts:
	src/com/android/mail/utils/NotificationActionUtils.java

Change-Id: I52a56282f632198e33603d44eb8dd1117dc76049