Floss: expose DBus method to allow HFP state query

Upon the audio stack invoking StartSCOCall, this method will be used
to periodically query the HFP state until it has transitioned to the
'Connected' state so that the audio stack can start sending data over.

Bug: 241050004
Tag: #floss
Test: Build and verify DBus method exposed
Change-Id: Idfe87944e7f2cf9916ab7dd9260305bea9634db6