Mms: update new messages notification count

Mms should notify the system about how many new sms/mms has (including those ones under conversations)
to the notification system. This allow the system to display the number of events for Mms app according its
settings

Change-Id: I15953283ae657b770869f611a886452199550d67
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>