Mms: Use a more familiar 'mark as read' icon

The existing 'mark as read' icon is a bit confusing and doesn't make clear
where it is for. This commit replaces the existing cion with the read icon
which is used in Email and Gmail.

Before: http://goo.gl/GXKfj
After:  http://goo.gl/RTEwa

Change-Id: I3c7ae7aa6173e29e687a7e0d564b2d54fcc61a51