All I would like to do
Keep in mind that there are many (many) layers of software, drivers, firmware, and even hardware at play to make an Arduino work with a PC. Many of those layers have over 10 years of legacy attached to them. Tablet operating systems don't necessarily carry of that legacy with them.
connect my Uno to my Motorola Xoom (via usb , as I have the attachment with the USB port on the end, like the camera attachement).
If your Xoom doesn't have drivers for a USB to Serial device (which is what the Arduino appears as to a host) then connecting it won't do any good. It is possible it does, but I don't know enough about the Xoom's implementation of Android to know that answer. Besides, unless you are running an application that takes to the serial port, what would you expect your Xoom to do when an Arduino is connected?
Once connected I just need my "Android App" to turn on and turn off a relay connected to my UNO. I thought it would be possible to do all of this WITHOUT a USB host shield!...
I'm not sure I understand where a host shield comes into play? A host shield would be used so that you could connect USB devices to an Arduino. In this case your Arduino is the device and the Xoom is the host.