Bluetooth: MAP: Cleanup old subfolder list on every folder listing

Older subfolders were still persisting , even if they are deleted,
renamed at the email server in absence of cleanup at every folder
listing cycle. Hence add change to cleanup the subfolders list
whenever folder listing is requested by remote

CRs-fixed: 590708

Change-Id: I37a72370bf1e3ea0e07012c43c9e29a176a998b1