Mms: Updated 'Mark as read' notification action icon

Users still think the 'Read' action means 'to read' (sounds like reed)
rather than 'the message has been read' (sounds like red).  This new
icon is an attempt to clear up the confusion and replaces the email
looking icon currenty in use by something a bit more suitable for the
Mms app.

Drawable curtesy of Björn Lundén

Screenshot:
  https://dl.dropbox.com/u/50993944/Screenshot_2013-04-08-18-16-03.png

Change-Id: I890a99c76173dfcb1179fea8ef80d24ecf5133ac
Merge branch 'cm-10.1' of github.com:CyanogenMod/android_packages_apps_Mms into cm-10.1