[Private GATT] Make datastores per-service

Previously the GattDatastore was global. Now, it can be specified
per-service, so that multiple datastores can be used.

This is needed to support GAP/GATT/DIS services, as well as non-isolated
connections.

Test: unit
Bug: 255880936
Change-Id: I581fec49621ce1cc2652b013b5978d659166fb95