Externalized the y-offset of undo popup to a dimensions resource

The plugin now reads the location of the undo popup's offset from bottom from a dimensions resource file. 
Users can override this by define the `elv_bottom_offset` dimension value in their projects. 

I use the `elv` prefix so users don't accidentally override the value (as is possible with other resources currently).

The offset is from the bottom of the screen. I didn't think changing the gravity to center for example, would be of any benefit and would also not be in-line with Android design. The use case for this is probably similar to mine in Issue #25 where the current popup overlaps some UI component.