Allow to change audio-route path to BT-headset also after receiving the acknowledgeme...

Allow to change audio-route path to BT-headset also after receiving the acknowledgement for bt_ag_open_sco.
Fix is for TDIS-6026, TZSP-6058

When switching from receiver- or speaker-audio path to BT-headset in PR3, changing audio-route path to BT-headset
fails in voicecall_snd_change_path(), because VCONF-value  'memory/bluetooth/btsco' will be set by BT-framework
to TRUE just AFTER checking it, so too late.

In Lunchbox, 'memory/bluetooth/btsco' will be set earlier, so voicecall_snd_change_path_real()
will be called in voicecall_snd_change_path().

Later even after succesfull receive of __vc_bt_ag_sco_state_changed_cb()-event, 'bt_connected'-flag is still
FALSE (because being earlier in receiver/speaker), so audio-path will not be changed even in this function.

'bt_connected'-flag will be set after __vc_bt_audio_connection_state_changed_cb() event,
but also now audio-path change fails in __vc_bt_handle_connectivity_event(), because it will be done only, if 'bt_connected' is FALSE.

So change of the audio-route never succeeds in PR3 after being earlier in receiver/speaker!