Restrict visibility of EXTRA_INSTALL_ZIPS

After aosp/3089654, Android.mk files will be able to read
EXTRA_INSTALL_ZIPS, which we don't want. Use the new
$(KATI_visibility_prefix) to restrict its usage.

Test: m nothing passes, adding $(warning $(EXTRA_INSTALL_ZIPS)) in build/make/core/main.mk produces an error
Change-Id: Ib3c0079efd534e580a176fc69f746852b66297af

android/makevars.go

@@ -513,6 +513,7 @@ EXTRA_INSTALL_ZIPS :=
 		fmt.Fprintln(buf)
 	}
 	fmt.Fprintf(buf, ".KATI_READONLY := EXTRA_INSTALL_ZIPS\n")
+	fmt.Fprintf(buf, "$(KATI_visibility_prefix EXTRA_INSTALL_ZIPS,build/make/core/Makefile)\n")
 
 	for _, symlink := range symlinks {
 		fmt.Fprintf(buf, "%s:", symlink.to.String())