Don't write to /sys/class/android_usb/android0/enable with configfs.

USB configfs doesn't use /s/c/a/a/enable. Trying to set that gives
confusing message on screen when sideloading.

Bug: 37713851
Test: adb sideload on device using with configfs.
Test: adb sideload on marlin.
Change-Id: Ifa55f90c2a5fe6bf9e7cee95882de9f6de686d73