Building Crosswalk: Update link to Chromium's AndroidBuild page.

The Install_prerequisites anchor does not exist anymore, so just point
to the page without any anchors.
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).
Merge pull request #118 from rakuco/update-android-build-instructions

Building Crosswalk: Update Android build instructions.