[audio][usb] Update USB audio drv proto

Update USB audio streams to support reporting unique IDs as well as
manufacturer/product strings. Also add support for controlling AGC
capabilities.

Test: Manual; Intel NUC with various USB audio adapters and the new
audio app.
Change-Id: I22aa41215c4f2028246f6bab1850df9e77d409c6

[usb][midi] Wire up USB MIDI interfaces again.

Change-Id: I7fe1b93049e57ba41ea1fc761ac883908d9f1cb2

[usb][audio] Update UsbAudioStream to use UsbAudioStreamInterface.

Update the UsbAudioStream class the new UsbAudioStreamInterface. The
stream class is in charge of publishing and implementing the fuchsia
audio stream driver interface, while the stream interface class is
concerned with exposing the controls needed to manipulate the USB
audio specific aspects of things. In particuilar, it exposes the
audio path as well as its gain controls, as well as configuration
required by an alternate streaming interface's endpoint (specifically
configuring the frame rate).

Also update the new UsbDevice class to take all of the steps needed to
create a stream interface and hand it off to the stream class. In
particular, the device handles the process of...

1) Discovering the control interface.
2) Discovering all of the terminals and units contained in the control
interface.
3) Traversing the terminal/unit graph to identify and build audio
paths that we care about.
4) Discovering all of the audio streaming interfaces and building our
their collections of alternate interface settings and supported
formats.
5) Linking paths to streaming interfaces, and transfering control of
streaming interfaces to the stream objects.

Change-Id: I5170878d7647bee1e9489aff6d2702fd84f290a3

[usb][audio] Begin to reorganize the USB audio driver.

Introduce a top-level USB device object which will be the owner for
all published streams. Update the old logging macros to make use of
'zxlogf' instead of using printf.

Change-Id: I8165962873293999519908a50d9f97c9973f5736