Call BTA_GATTS_AddService on correct thread

Right now BTA_GATTS_AddService is incorrectly called on btif thread.
This might lead to situations where it is executed while bta thread is
already cleaning up, and deleted the control blocks, resulting in crashes.
From now on, execute it on bta thread. This ensures that it is executed
before control blocks are freed.

Test: sl4a GattClientTest
Bug: 73054849
Change-Id: Ie2bf80414bceba2590c21d3825c78fbb58449520