DO NOT MERGE MAP: fixed missing filtering on Priority

FilterPriority was ignored in the code as SMS does not have priority.
This fixes the support for the priority filter for sms + mms.
NOTICE: sms messages does not have priority and hence will return
an empty messagelisting if the client requests a list with priority
filter set til high-pri only.

Bug: 11168636
Change-Id: If028e47850323d20d1e9a23bc947782ae8da1df0