Fix test-art-target-sync when ART_TEST_ANDROID_ROOT is used.

The semantics of "adb push" have changed since the time
system/core was pinned in the master-art manifest.

Bug: 26051370
Change-Id: I3ee6f2694168b0cdc8c8ac37f77fd443879275dc
diff --git a/Android.mk b/Android.mk
index 0d0003a..3438beb 100644
--- a/Android.mk
+++ b/Android.mk
@@ -146,7 +146,10 @@
 test-art-target-sync: $(TEST_ART_TARGET_SYNC_DEPS)
 	$(TEST_ART_ADB_ROOT_AND_REMOUNT)
 	adb wait-for-device push $(ANDROID_PRODUCT_OUT)/system $(ART_TEST_ANDROID_ROOT)
-	adb push $(ANDROID_PRODUCT_OUT)/data /data
+# Push the contents of the `data` dir into `/data` on the device.  If
+# `/data` already exists on the device, it is not overwritten, but its
+# contents are updated.
+	adb push $(ANDROID_PRODUCT_OUT)/data /
 endif
 endif