HF Client: SCO routing with multiple devices

For SCO it makes sense only to have one active connection at a time for
any device since simultaneous SCO calls in a single device is
un-reasonable. This change changes the SCO state machine in following
ways:
a) SCO is not setup automatically whenever AG wants, the client for this
BTA layer will have to call connect_audio to start SCO
b) Also when we disconnect SCO for a device, it does not go into a
listen state. We are making sure that the upper layer caller always has
to call connect to accept a connection

Bug: b/34518638
Test: Manual testing w/ multiple active calls and SCO switching

Change-Id: I6e6c4c6e6ee19e44e3ff14499fb99083102bde5a