Add stack/btm/btm_sco::GetLegacyHciInterface

Issue hci disconnect in stack/btm/btm_sco::BTM_RemoveSco

No need to bother the acl layer since it knows nothing
about it anyway.

Bug: 177382279
Tag: #refactor
Test: gd/cert/run

Change-Id: I8704754d0a8737b183e3e5e3892604b209b0aefa