Bluetooth: MAP: Not doing Message listing when proper folder is not set

Add change to do message listing of any folder, only if it is a
subfolder of the current folder and not do message listing otherwise.
Without this change we were able to do message listing of any folder
irrespective of the current folder.

Change-Id: I1c2e703b287ab61fdfb1e852ea82c4d4174bc70b
CRs-fixed: 577982