The standard phrase in the US is "presumed innocent." Lawyers would use it as a stock phrase, also in noun form: "presumption of innocence." "considered innocent" as a phrase just looks a little wrong. You could use "considered" in other contexts, such as, "There hasn't been a trial yet, so we have to consider him to be innocent." The grammar would be fine, and there is no real difference in meaning. However, this might suggest a non-native writer or speaker, who doesn't know that the standard phrase is "presumed innocent."
To me, "presumed" in the legal context carries the distinct implication of doubt and "considered" has the connotation of somewhat more confidence.
In the real world, if courts REALLY treated everyone as though they were innocent, even thouse accused of the most heinous crimes would not be either incarcerated or required to post bond pending trial.