[Runtime] Small refactor to our PathProvider

Move the logic for xwalk::DIR_DATA_PATH to a separate function, and
create a helper function for Linux for getting the config path from
XDG. These will be useful when we transition Tizen to use home directory
for service mode instead of the "/opt/usr/apps".
Merge pull request #1409 from cmarcelo/small-refactor-xwalk-paths

[Runtime] Small refactor to our PathProvider