libbinder_ndk: stability product build var canary

Build define to ensure that vendor stability is not
used on the product partition.

Bug: 385007343
Test: N/A
Change-Id: I3fd03dde807b64e2c96c4f86cf10d619eb41b32e

Author: Steven Moreland

libs/binder/ndk/include_platform/android/binder_stability.h

@@ -27,6 +27,10 @@ constexpr binder_flags_t FLAG_PRIVATE_VENDOR = 0x10000000;
 
 #if defined(__ANDROID_VENDOR__)
 
+#if defined(__ANDROID_PRODUCT__)
+#error "build bug: product is not part of the vendor half of the Treble system/vendor split"
+#endif
+
 /**
  * Private addition to binder_flag_t.
  */