Unify LE advertising instance identifiers

Up till now, we had three numbers associated with each advertising
instance: client_id, inst_id, and cbindex. We also had special structure
mapping each of those numbers to another.

From now on, when registering advertiser, we'll grab next free
advertising instance id and make advertiser_id equal to it. Thanks to
this, we can remove all the mapping and have just one number associated
with each advertising instance.

This also means we no longer need to pass *p_ref value to the BTM layer
with each request, as advertiser_id is equal to inst_id.

Bug: 30622771
Change-Id: Ied71bff36e30d6c6ce4ca3e62d46ba96320cf597