API: created a new api to flag messages as read

New api is called "ClearHasMsg" and does nothing but erase a new message
flag from currently logged in user.

This is useful for tools that can read the new messages using some api, or some
other way (loading the text using different session, preload the text using a
buffer where it's unrevealed later if user actually did read it, or closed the
application before), so it can be useful in situations when you need to flag new
messages as read in a different time than that when you actually read them.

Bug: 64238
Change-Id: Ife575711c32bb8e3bcac789de4a6b37e1888d032
Merge "API: created a new api to flag messages as read"