apidiff: Depend on timestamp file instead of phony target

Instead of $(LOCAL_MODULE)-docs, use $(call
doc-timestamp-for,$(LOCAL_MODULE)), which is how the *-docs targets are
generated in droiddoc.mk

Test: m offline-sdk-referenceonly-diff
Change-Id: I1e5ee3b819c0e836089fac236e977642067d148a
diff --git a/core/apidiff.mk b/core/apidiff.mk
index 180aabd..7876c11 100644
--- a/core/apidiff.mk
+++ b/core/apidiff.mk
@@ -124,7 +124,7 @@
 		$(full_java_lib_deps) \
 		$(jdiff) \
 		$(doclava) \
-		$(LOCAL_MODULE)-docs \
+		$(call doc-timestamp-for,$(LOCAL_MODULE)) \
 		$(LOCAL_ADDITIONAL_DEPENDENCIES)
 	@echo Generating API diff: $(PRIVATE_OUT_DIR)
 	@echo   Old API: $(PRIVATE_OLDAPI)