Set OS=android via GYP_DEFINES

Otherwise, the variable isn't available in all scopes. Instead of playing around
with nested scopes, we just set it in GYP_DEFINES (same as chromium does)

BUG=v8:3405
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/357643002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22004 ce2b1a6d-e550-0410-aec6-3dcde31c8c00