Email: Notify UI data changed of the unread mailbox.

Notify UI the data has been changed of the unread mailbox and UI will
update the content according to the new data.

CRs-fixed: 630974

Change-Id: I041d2638c8c4c892e6207cdac7dd89edf34d511f
Merge "Email: Notify UI data changed of the unread mailbox." into du44