HFP: Reduction in SCO switching time

During the call, when the HF active device is
changed, the SCO switching takes more time due
to multiple device switching that is during
active device change, SCO disconnect and SCO
connect.

This can be avoided by adding dummy device
address into the connected device hashmap when
the active device is connected and removed from
the hashmap when the active device is removed.
Hence the SCO switching time is reduced.

CRs-Fixed: 2344990
Change-Id: Iac90cc07f59289c2318b743b31a148295bf758c3