From my experience with the Android UI, here's what i've noticed
- When using a Maps activity, there is really no reason to align to the right, you are looking at a map, ie. a 2D surface that can be scrolled at will.
When browsing a website, most content starts at the left and runs right.
So, placing the control in the lower right while browsing the web makes a lot of sense, to keep it out of the way, when using maps (where the zoom control is more important), the control should be more prominent, thus in the center, where it still does not get in the way here.
- Upon first reading your post, I was going to suggest using a custom zoom control, but I realize that you have thought about this already. If having centered controls is essential to your application, a custom control seems to be the route I'd take.