Add task for API diffs

Test: make api-diff
Change-Id: Ifb788069bf1d882a52e8429e056070675f866986
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 6ac2602..d2e7a6c 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -14,6 +14,8 @@
 LOCAL_ALLOW_UNDEFINED_SYMBOLS:=
 LOCAL_ANNOTATION_PROCESSORS:=
 LOCAL_ANNOTATION_PROCESSOR_CLASSES:=
+LOCAL_APIDIFF_NEWAPI:=
+LOCAL_APIDIFF_OLDAPI:=
 LOCAL_APK_LIBRARIES:=
 LOCAL_ARM_MODE:=
 LOCAL_ASFLAGS:=