Add a selected UsbAlsaDevice

Keep track of the currently selected UsbAlsaDevice in a
synchronized manner.
Allow it to notify AudioService that it has been disabled
prior to replacing it with a new Audio Device.

Test: verified audio worked as expected when using
both UAC2 and non-UAC2 3.5mm adapters, with 3.5mm
plugging/unplugging and USB-C plugging/unplugging.

Change-Id: I8b76145490da38d4eec25f4f2bdb6695afa151b9