bass_client: Use PA_Sync value in update source context

Intention of this change is to unify usage of parameters in modify
source context. To not mix, status with value let's use a most proper
value which is PA_Sync value.

Tag: #bug
Bug: 330690507
Bug: 333050419
Test: atest BassClientServiceTest
Test: atest BassClientStateMachineTest
Change-Id: I2125994b7251b9da2abb5b7298b78ceafe52a7e7