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).