Bluetooth: Cleaning up cursor object to avoid memory leak

A case where cursor objects are leaked after BT OFF/ON,
when it is referenced (Directly OR Indirectly). So Garbage
Collector doesn't collect cursor objects which are referenced.
Setting NULL to cursor object after closing to mark those
variable as weak reference object. Any objects memory is
immediately claimed by GC, which are marked as weak Reference.

Change-Id: Ie1665c9c9ff69b2a190025d508aa9924b36146dd
CRs-Fixed: 579016