Add support for Multi-A2DP state machines per device

Update usage of A2dpService API calls that take BluetoothDevice
as an additional argument. If the BluetoothDevice argument is null,
the API applies to the device that is currently the Active A2DP device.

Exempt-From-Owner-Approval: De-facto owner of the relevant changes is
the Bluetooth team.
Bug: 69269748
Test: Manual
Change-Id: I7417b7b0741f706df475cb2b27fbe6525f744269