Switch tar to zip for factory images

BUG:31142410
Change-Id: Ib396ca3bdc8c4e27f666b7c93d580ac2bb57021b
diff --git a/generate-factory-images-common.sh b/generate-factory-images-common.sh
index b8d6707..dc1bfcd 100644
--- a/generate-factory-images-common.sh
+++ b/generate-factory-images-common.sh
@@ -293,8 +293,8 @@
 chmod a+x tmp/$PRODUCT-$VERSION/flash-base.sh
 
 # Create the distributable package
-(cd tmp ; tar zcvf ../$PRODUCT-$VERSION-factory.tgz $PRODUCT-$VERSION)
-mv $PRODUCT-$VERSION-factory.tgz $PRODUCT-$VERSION-factory-$(sha256sum < $PRODUCT-$VERSION-factory.tgz | cut -b -8).tgz
+(cd tmp ; zip -r ../$PRODUCT-$VERSION-factory.zip $PRODUCT-$VERSION)
+mv $PRODUCT-$VERSION-factory.zip $PRODUCT-$VERSION-factory-$(sha256sum < $PRODUCT-$VERSION-factory.zip | cut -b -8).zip
 
 # Clean up
 rm -rf tmp