Fix null pointer exception if USBPort is removed

If for some reason a USB port instance is removed, UsbPortManager
handles this by broadcasting a UsbManager.ACTION_USB_PORT_CHANGED
intent with null as the UsbPortStatus argument, leading to a null
pointer exception after UsbDeviceManager receives it. Take care of
this by checking for null in UsbHandler.updateHostState().

Bug: 156230008
Change-Id: Ia8936358c61af5b537d1eb722a55851e0061b381