Re-implement SMS/MMS Mark all as Read option.

Change-Id: I199207441dc37393d733e3af443730367d352f2e
Make "Mark all as Read" context sensitive

This commit make the mark all as read option only show
when you have unread conversations and not at all times
similar to the behavior of delete all threads option.

Change-Id: I0de7967f14f67b4938a32afccdf05d033a5599aa