BT: Don't switch devices when one of the connected BT headsets disconnects

Two BT headsets, hs1 and hs2 are connected to the device and
a voice call is made. Call audio is present on hs2. If hs1
is disconnected, we should not switch the audio to handset
or speaker.

CRs-Fixed: 968437
Change-Id: I4108c7ab810f9b56a95f820c99b2b981a9a17157