audio service: fix regression in startBluetoothSco

Commit 2a57ca93 introduced a regression in startBluetoothSco()
where the calling pid was cleared before creating the entry for
the client app. The pid in the entry was always the system server pid
and the SCO client verification logic was broken preventing the activation
of the BT SCO connection.

Change-Id: I4e024b22fceb350f829ff0d8664703faeef7af48