This question has no firm answer.
Weeping Angels are quantum locked. They are forced into their stone form whenever they are being viewed. The mechanism is unknown, but it's implied that the only exceptions are mirror images of the angels themselves -- even a video camera set to record (or another angel!) appears to do the job. Also, they would appear to have a stonelike texture even out of the lock -- the few times we are shown a moving angel, it retains its stony appearance.
Now, to your question: the initial rules given to us in "Blink" was that Weeping Angels were not able to move while stone, and that they had no choice in the matter; the very act of them being viewed would turn them into inanimate stone. In other episodes with Weeping angels, this rule was clearly in force. In "Flesh and Stone", however, this is clearly not the case.
Both episodes are written by Moffat, who is a notoriously bad writer when it comes to consistency in-universe. By Occam's Razor, I declare that Weeping Angels moving while in stone form is the fault of Moffat's inconsistency and, as such, cannot be declared true or false, as his writing consists of performing Shrodinger's experiment on plot.