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
Signed-off-by: Neill Kapron <nkapron@google.com>
(cherry picked from https://android-review.googlesource.com/q/commit:5157908192488f762877e785131f56664fbcff61

)
Merged-In: Ic1dfc6b61f94a6ac2f78e41edf4f87730f73e00e
Change-Id: Ic1dfc6b61f94a6ac2f78e41edf4f87730f73e00e
[nkapron: resolve merge conflict in drivers/usb/gadget/Kconfig]
Signed-off-by: Neill Kapron <nkapron@google.com>