In kAwaitingUsbConnection, we have called function_.SetInterface(this), and
are waiting for the function driver to call SetConfigured(true). Calls to
SetConfigured(false) are ignored.

Once SetConfigured(true) has been called, we:
- Send USB "receive" requests to the endpoint,
- Tell any connected UsbAdbImpl clients that the device is online, and
- Move to kOnline.

If a fadb::Device client requests shutdown by calling StopAdb() or closing
a UsbAdbImpl connection, we call function_.SetInterface(nullptr), and move
to kStoppingUsb. Likewise if PrepareStop is called.

In kOnline, the USB connection is live, and we respond to any
QueueTx/Receive requests from UsbAdbImpl clients.

If any of the following happen:
- A fadb::Device client calls StopAdb().
- A UsbAdbImpl client closes their channel.
- The USB function driver calls SetConfigured(false)
- PrepareStop get called.

... we call function_.SetInterface(nullptr) and move to kStoppingUsb. (We
hold onto the responder for any calls to StopAdb()).

In kStoppingUsb, we wait for all outstanding USB requests to be completed.
Once they have been, we:
- Return OK to any StopAdb() calls that triggered the stoppage (or
happened while in kStoppingUsb).
- Tell any connected UsbAdbImpl clients that the device is
offline,
- Close all UsbAdbImpl connections.

At that point, if the stoppage was caused by a call to PrepareStop (or
PrepareStop we called while shutting down), we respond that the driver has
shutdown successfully. Otherwise, we restart the USB connection by calling
function_.SetInterface(this), and move back to kAwaitingUsbConnection.