Add additional GattServiceTest #2

The preparation of testability of `GattService` from aosp/2164121 is not available in `tm-qpr-dev` (due to a missing cherry-pick), some functionality was not available to be fully verified. This cherry-pick differs slightly from aosp/2375750 with those verification lines being omitted.

Bug: 237467631
Test: atest GattServiceTest
Change-Id: I41631380f4de987b57154194741151e0cfe1ddad
(cherry picked from commit 686aeff3)
Merged-In: I41631380f4de987b57154194741151e0cfe1ddad