Multiple USB support.

By reading system property ro.audio.multi_usb_mode to decide if only one
or multiple usb devices can be connected to audio system at a certain
time.
In multi USB mode, it is allowed to connect multiple USB devices to
audio system as long as their device type is different.

Bug: 262415494
Test: manual
Change-Id: I10d6ea14188105b4cd96ed0973dbea1f3e80a852