tests: Bluetooth: Tester: Refactor ACSC events for BAP unicast

This commit refactors when the BTP_ASCS_EV_OPERATION_COMPLETED
is sent to the upper tester. Previously it was send
in the stream changed callbacks, but that would send
the events to the BAP Unicast Server when it did not initiate
any changes, and would not properly match when the control
point operation was completed (which happens before the
ASE state changes).

This change makes it easier to determine when an operation was
actually completed, and easier to separate that from a
ASE state change which is another type of event.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>