Add warning for legacy VNDK use

Trying to curb propagation of this POC.

Bug: 69526027
Test: N/A
Change-Id: I02000477520951939e5ded6c1c55d6aa644e0b35
diff --git a/vndk/Android.mk b/vndk/Android.mk
index 48a72f4..b112c71 100644
--- a/vndk/Android.mk
+++ b/vndk/Android.mk
@@ -1,3 +1,6 @@
+# TODO(b/69526027): DEPRECATE USE OF THIS.
+# USE BOARD_VNDK_VERSION:=current instead.
+
 ifneq ($(filter generic%,$(TARGET_DEVICE)),)
 
 LOCAL_PATH := $(call my-dir)