Le callback: Pass gattService from app directly

This allow to avoid getting the gattService in async from the framework
context and ensure the gattService is not null.

Bug: 318461868
Test: m com.android.btservices
Flag: EXEMPT, no logical change only passing back a variable instead of
      getting it later Talked offline with Hallstrom about the flagging
      need
Change-Id: I1bd114564742253a86d6eced1f0db16ee59dad57