usb: gadget: android: Convert to new audio_source interface

The audio_source driver was recently converted to conform to
the USB_FUNCTION API. This means android.c should not include
f_audio_source.c directly but instead should obtain an instance
of it using the usb_get_function_instance/usb_get_function APIs.

Signed-off-by: Jack Pham <jackp@codeaurora.org>