Add LegacyUsbDeviceManager::setCurrentFunction method.

Method implementation is the same as UsbDeviceManager::setCurrentFunction.
The override is required to use LegacyUsbDeviceManager's mHandler, as
UsbDeviceManager's mHandler is an uninitialized private field.

Change-Id: I9ffece5247c17de77fa799e1f6c9b439ebd074db