Fix the header user agent constant in core

[cammace]

HEADER_USER_AGENT in the core Constants class is not using its traditional format that we had in previous versions. This is because in the current setup, we are not generating Build Config file which stores information that is needed. We were using a variation of this project.

https://github.com/mapbox/mapbox-java/blob/master/services-core/src/main/java/com/mapbox/services/constants/Constants.java#L19

cc: @zugaldia