shill: simplify and clean up GeolocationInfo

GeolocationInfo is a thin wrapper of `std::map<std::string, std::string>',
which doesn't provide value in terms of abstraction or functionality.
This CL simply turns it into a simple type alias.

BUG=None
TEST=Run unit tests.

Change-Id: Id56e2de448c9c3f0e101208d3796e50ed1edfaf2
Reviewed-on: https://chromium-review.googlesource.com/780445
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>