Add gradle target 'downloadDx'.

Bug:
Change-Id: Ic4fc5db7d3910167da71dc20dab542dadea4fc81
diff --git a/build.gradle b/build.gradle
index a3a6419..419bbae 100644
--- a/build.gradle
+++ b/build.gradle
@@ -195,6 +195,16 @@
     }
 }
 
+task downloadDx {
+    cloudDependencies.each { entry ->
+        entry.value.each { entryFile ->
+            if (entryFile.contains("dx.tar")) {
+                dependsOn "download_deps_${entry.key}/${entryFile}"
+            }
+        }
+    }
+}
+
 task downloadAndroidCts {
     cloudDependencies.each { entry ->
         entry.value.each { entryFile ->