Feature request: Add anchor option to scale bar plugin

On iOS, one can use `self.mapView.scaleBarPosition = .bottomRight` to automatically position the scale bar in the bottom right. It would be nice to have a similar option on Android, rather than manually setting margins.

Start by locating the scale bar plugin and inspecting its current margin-based positioning. Use the iOS bottom-right example in the issue as the behavioral reference. Done should mean Android users can select an anchor such as bottom-right instead of manually setting margins.

Written by the indexing model from the issue text.