Unify code in scanner/advertiser connection complete handler.

btm_ble_set_conn_st is called in every possible scenario of
l2cble_scanner_conn_comp. Call it at start, instead of before each
return.

Test: sl4a GattConnectTest
Bug: 112827989
Change-Id: I969fc59b934dc1c7a52be00741fdf38402545b26