HFP AG: Fix the condition to check if SCO is connected

Two headsets hs1, hs2 are connected and SCO is created with
hs2. If hs1 is disconnecting, we move to MultiHFPending state
from AudioOn state and then move back to AudioOn state. We
should check for audio state and SCO device instead of
checking if we are in AudioOn state.

Test: 1. Create connection from DUT to HS1
2. Create connection from DUT to HS2
3. Make MT call on DUT and accept from DUT
4. Now disconnect HS1 from DUT

Bug: 63740200
Bug: 35657481
Change-Id: I4956541ec234d6ebc76a8b1478b7caa132c44605