[Private GATT] Enforce NRPA for private GATT

As per design at go/private-gatt-in-platform, all advertisements using
an isolated server must use an NRPA. This adds a check for that in the
service.

Test: CTS multi-device
Bug: 274945531
Change-Id: If564983b3b3d5ff2b3f883151f47f8f7026e8689