[GATT Server] Use TransportIndex instead of conn_id in API

In preparation for supporting multiple GATT servers simultaneously, the
API of GattModule should use TransportIndex (i.e. connection handle),
rather than ConnectionId, since multiple ConnectionIds exist for a
single TransportIndex.

Test: unit
Bug: 274945531
Change-Id: Ie820bdab7a0d14b7f0aa6f2db37aa3bf568f4242