Support to build native libraries with prebuilt NDK

Bug: http://b/issue?id=2811253
Change-Id: Iabd4ab8b3d7236dffe89f1939e1142e882944026
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 0fe3c8b..75297bc 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -88,6 +88,7 @@
 LOCAL_DX_FLAGS:=
 LOCAL_CERTIFICATE:=
 LOCAL_SDK_VERSION:=
+LOCAL_NDK_VERSION:=
 LOCAL_NO_EMMA_INSTRUMENT:=
 LOCAL_NO_EMMA_COMPILE:=
 LOCAL_PROGUARD_ENABLED:= # '',optonly,full,custom