Building Crosswalk: Update Android build instructions.

* Recommend using a `chromium.gyp_env` file just like upstream to set
  variables like `target_arch` and `OS`.

* Always recommend setting `OS=android`. That is not strictly necessary
  until Crosswalk 7, but it has since become mandatory because the
  `envsetup.sh` script has stopped doing it implicitly.

* Unify the instructions for calling `xwalk_gyp_android`: if one sets all
  variables in `chromium.gyp_env`, the same command can be called for all
  currently used Crosswalk versions (5, 6 and 7).