Monotonicity for NEED_AIDL_NDK_PLATFORM_BACKEND.

Hint to partners this will go away. We have limited
testing of this option, and we shouldn't maintain two
ways of handling NDK AIDL variants. So, this way
we can remove it in the future. :D

Bug: 161456198
Test: build
Change-Id: I5ae43b4f1b6c0d7253e82caeb338b69b7e00fdb6
diff --git a/core/config.mk b/core/config.mk
index 0ecee8a..0cf7eb9 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -811,6 +811,12 @@
   BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED ?= true
 endif
 
+ifneq ($(call math_gt_or_eq,$(PRODUCT_SHIPPING_API_LEVEL),36),)
+  ifneq ($(NEED_AIDL_NDK_PLATFORM_BACKEND),)
+    $(error Must not set NEED_AIDL_NDK_PLATFORM_BACKEND, but it is set to: $(NEED_AIDL_NDK_PLATFORM_BACKEND). Support will be removed.)
+  endif
+endif
+
 # Set BOARD_SYSTEMSDK_VERSIONS to the latest SystemSDK version starting from P-launching
 # devices if unset.
 ifndef BOARD_SYSTEMSDK_VERSIONS