Add Bluetooth headset API to allow disabling audio route.

This functionality is required by devices which have multiple profiles
using the SCO channel. For example, a device that is both a HFP AG and HF.
In this case, we must explicitly notify the profiles when they can acquire
the channel as they are not aware of each other. A similar change was
previously added to the Bluetooth Headset Client profile.

Bug: 25485578
Change-Id: Ia60cfdd33c2c3c3f185464b24056f8ccb976056d