android usb gadget: reenable the enable/disables for functions

Using the standard Android mechanism, so the userspace in
system/core/rootdir/init.usb.rc works.

N.B.: This disables USB_ANDROID_USBNET in android.c, but nowhere
else.  Should probably be cleaned up at some point; that driver
is not used by stock Android, which uses rndis.

Change-Id: Ic291988275385fa3bf077a6329ebdfbb2bff4562