[Android] fixed android build issue

As upstream bumps to version 36.0.1985.18, we found we always
failed to build xwalk android as we haven't pass -DOS=android to
gyp_xwalk. So added it into build/android/envsetup.sh.

Also removed '--check' because if we pass -DOS=android to
gyp_xwalk, the '--check' will be append into args.