android: only emit multilib variables for target builds.

Host modules shouldn't set LOCAL_MODULE_TARGET_ARCH or other multilib
build related variables, and shouldn't use multilib intermediate
directories. This wasn't breaking anything as it was a no-op, but
upcoming android build system changes will cause this to do the wrong
thing.

BUG=
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/262603003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1910 78cadc50-ecff-11dd-a971-7dbc132099af