Add a missing include in DocumentsUI Android.mk.

Without it, perf tests were not built on APCT.

Bug: 27370274
Change-Id: Ie154526d240bb9d1b4cce6fc089d502bf808c366
diff --git a/Android.mk b/Android.mk
index 6dfc3bb..d5e48b5 100644
--- a/Android.mk
+++ b/Android.mk
@@ -38,5 +38,4 @@
 LOCAL_CERTIFICATE := platform
 
 include $(BUILD_PACKAGE)
-
-include $(LOCAL_PATH)/tests/Android.mk
+include $(call all-makefiles-under, $(LOCAL_PATH))