Floss: expose DBus method to notify HFP audio disconnection

Currently there is no reliable way for the audio stack to learn if HFP
audio has been disconnected by the headset.

Through experiments, we have learned that in such case the headset is
almost always ready for reconnection, so it is critical to pass this
information for the audio stack to handle properly.

Bug: 241527929
Tag: #floss
Test: Build and verify DBus method exposed

Change-Id: I87f9842cd0a089e326caa5f4cfeeb74171a2a187