MHF: Broadcast connected state for HS

- Since telephony doesn't store list of connected HS, BT needs
to broadcast connected state for an already connected HS,
when other HS disconnects. This will notify telephony that
even though one HS is disconnected, but still there is one
other connected HS. This fixes call routing issue to BT HS
from phone UI.

- Set mActiveScoDevice to null when HS disconnects while
it has SCO connected for ongoing call.

Change-Id: I91aba22844938b61afc0bac52b5b6de95354f491
CRs-Fixed: 619841