Support 'center' alignments in ToastManager

[bploeckelman]

Looks like bottom, top, left, right and their combinations are the only supported Toast position alignment values. Other UI widgets are often stuffed into screen corners and end up fighting with Toast visibility so it would be helpful to also support center, though possibly just for the bottom and top edges.

At the very least, changing ToastManager.updateToastsPositions() to protected visibility would allow end-users to customize positioning behavior themselves just by extending ToastManager and overriding the method rather than having to copy the whole thing into a customization class in order to change that behavior.