Rework use of ~ in build.json

Based on:
https://github.com/apache/cordova-cli/pull/359
https://github.com/apache/cordova-android/pull/563
https://github.com/apache/cordova-docs/pull/909

Conclusion was to not merge any of those PRs, but rework how the support for `~` is implemented. Instead of manually reading env variables, using `untildify` should be much more stable.

During the discussion also the suggestion came up to support this in all `build.json` files, not just for `cordova-android`. (Will have to check if any of those use paths at all, so that it is worth the effort to move this from `cordova-android` to a more general location.)