UsbService: Make USB disconnect handling for accessory mode more robust

Handle the case where the kernel driver is in accessory mode but we failed
to initialize it at the framework level.  On disconnnect, check to see if the
accessory kernel driver is enabled rather than checking mCurrentAccessory.
That way we will restore the USB state in the kernel even if mCurrentAccessory
is null.

Change-Id: I35d458f21a8b21611946da523d0f53723cab0540
Signed-off-by: Mike Lockwood <lockwood@android.com>