Well, IANAL but 'sub-licensing' means that any right you give Google, that they have the right to licence those to others.
If you look at the GPL, you'll see it talks about sub-licensing but says it's unnecessary since the GPL gives the receiver all the rights of the GPL from the original author so there is nothing to sub-license. So AFAICS it has no effect on strong-copyleft either.
It's certainly not assigning copyright. I don't think it can because in several jurisdictions you can't do that with an electronic form anyway. My theory is that the Google CLA is designed to be able to cover anything, not just open-source stuff, and that its effect on non-open source stuff is more profound. Documentation you submit comes to mind.
Still, I'd like someone with legal training to actually write something (you know, with citations and research) about the interaction between contributor agreements and copyright licences, because it vaguely feels like a dual-licensing arrangement, which always seemed like a dodgy part of copyright law to me anyway.