Make aninja work with alternate out dirs

Test: Manually
Change-Id: Ia7b18b04cabafa0bbce0232b32547f6d87059dc4
diff --git a/bin/aninja b/bin/aninja
index 5acb968..5cb5a55 100755
--- a/bin/aninja
+++ b/bin/aninja
@@ -34,5 +34,5 @@
 esac
 
 cd $(gettop)
-prebuilts/build-tools/${host_arch}/bin/ninja -f out/combined-${TARGET_PRODUCT}.ninja "$@"
+prebuilts/build-tools/${host_arch}/bin/ninja -f $(getoutdir)/combined-${TARGET_PRODUCT}.ninja "$@"