PBAP: Intialise Service instance before START_LISTENER is processed

Usecase:
1) Turn ON Bluetooth.
2) Establish PBAP Connection.
3) Turn OFF Bluetooth.
4) Repeat above steps in loop.

Issue:
PBAP Connection fails rarely (1/50)

Root Cause:
When BluetoothPbapService is started, before PBAP Service instance
is intialized in setBluetoothPbapService(), START_LISTENER is
processed in PbapHandlerThread and null is passed ObexServerSockets
leading to incoming connection being rejected.
This looks to be regression introduced due to commit
bd90909c.

Fix:
Invoke setBluetoothPbapService() before START_LISTENER is sent.

Test: Connection didnt fail in reliability test.

Bug: 118473239
Change-Id: Ic530d520e3e5428b669398948dbc268840c46730