ANDROID: usb: gadget: f_midi: Create f_midi device in android_usb class

Android frameworks rely on the sound device configuration information
provided by this change.

Creates a device file for f_midi which, when read, prints the alsa card
number and device identifier in the format of "<number> <device>". If
the device is not configured, "-1 -1". is returned.

Bug: 120441124
Bug: 317149848
Change-Id: Ic1dfc6b61f94a6ac2f78e41edf4f87730f73e00e
Signed-off-by: Neill Kapron <nkapron@google.com>