Use var for design library

Bug: 72796206

Test: make checkbuild
Change-Id: I1452b440a04f4a594f0cf4dfbbf4192b5af5b881
diff --git a/Android.mk b/Android.mk
index 11cdd77..4a6a325 100644
--- a/Android.mk
+++ b/Android.mk
@@ -14,7 +14,7 @@
 LOCAL_ASSET_DIR := $(addprefix $(LOCAL_PATH)/, $(asset_dirs))
 
 LOCAL_STATIC_ANDROID_LIBRARIES := \
-    android-support-design \
+    $(ANDROID_SUPPORT_DESIGN_TARGETS) \
     android-support-transition \
     android-support-v13 \
     android-support-v7-appcompat \