Add PRODUCT_IOT support to soong.

Some code needs to behave differently for Android Things
systems. PRODUCT_IOT tracks Android Things products.
This change exposes this variable into the soong system.

Bug: 110494430
Test: aosp_angler builds
Change-Id: Id9c27e4fe70957f217cfbdc019e8a37532bafac2
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 355f414..f75e263 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -124,6 +124,8 @@
 $(call add_json_bool, Malloc_not_svelte,                 $(call invert_bool,$(filter true,$(MALLOC_SVELTE))))
 $(call add_json_str,  Override_rs_driver,                $(OVERRIDE_RS_DRIVER))
 
+$(call add_json_bool, Product_is_iot,                    $(filter true,$(PRODUCT_IOT)))
+
 $(call add_json_bool, Treble_linker_namespaces,          $(filter true,$(PRODUCT_TREBLE_LINKER_NAMESPACES)))
 $(call add_json_bool, Enforce_vintf_manifest,            $(filter true,$(PRODUCT_ENFORCE_VINTF_MANIFEST)))