Fix generating symlinks for older trees

Symlinks for unpigz, toolbox, and busybox were not generating in
older trees.

Change-Id: I3ca2a2047510954e66fb3cf49eef6af713d3268e
diff --git a/Android.mk b/Android.mk
index 130575f..e909baa 100644
--- a/Android.mk
+++ b/Android.mk
@@ -394,6 +394,9 @@
 LOCAL_MODULE := busybox_symlinks
 LOCAL_MODULE_TAGS := optional
 LOCAL_ADDITIONAL_DEPENDENCIES := $(RECOVERY_BUSYBOX_SYMLINKS)
+ifneq (,$(filter $(PLATFORM_SDK_VERSION),16 17 18))
+ALL_DEFAULT_INSTALLED_MODULES += $(RECOVERY_BUSYBOX_SYMLINKS)
+endif
 include $(BUILD_PHONY_PACKAGE)
 RECOVERY_BUSYBOX_SYMLINKS :=
 endif # !TW_USE_TOOLBOX