Add additional GattServiceTest

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/2373373 with those verification lines being omitted.

Bug: 237467631
Test: atest GattServiceTest
Change-Id: I2247d0bbc3de8e1d6a18f63ac0f1b4a4e9a2cfb4
(cherry picked from commit c3ebd225)
Merged-In: I2247d0bbc3de8e1d6a18f63ac0f1b4a4e9a2cfb4