Correct apex data in androidmk for flattened apex

Incorrectly used fmt.Println, printing to stdout instead of the desired
file.

Test: m com.android.art.testing
Bug: 155820504
Change-Id: I1aaf15de4f18f71710bc344bc8f71723e9756ad4
diff --git a/apex/androidmk.go b/apex/androidmk.go
index 09d3607..6b168fe 100644
--- a/apex/androidmk.go
+++ b/apex/androidmk.go
@@ -121,7 +121,7 @@
 				fmt.Fprintln(w, "LOCAL_MODULE_SYMLINKS :=", strings.Join(fi.symlinks, " "))
 			}
 			if len(fi.dataPaths) > 0 {
-				fmt.Println(w, "LOCAL_TEST_DATA :=", strings.Join(cc.AndroidMkDataPaths(fi.dataPaths), " "))
+				fmt.Fprintln(w, "LOCAL_TEST_DATA :=", strings.Join(cc.AndroidMkDataPaths(fi.dataPaths), " "))
 			}
 
 			if fi.module != nil && len(fi.module.NoticeFiles()) > 0 {