Use var for design library

Bug: 72796206

Test: make checkbuild
Change-Id: I534d3c01948d09d8f9172b8207e0268241ca6f19
diff --git a/Android.mk b/Android.mk
index ddfad3e..be49a71 100644
--- a/Android.mk
+++ b/Android.mk
@@ -121,7 +121,7 @@
 
 LOCAL_STATIC_ANDROID_LIBRARIES := \
 	android-support-core-ui \
-	android-support-design \
+	$(ANDROID_SUPPORT_DESIGN_TARGETS) \
 	android-support-transition \
 	android-support-v13 \
 	android-support-v4 \