Add flag to control hidden API warning toasts

Just like visual warnings about improper use of native libraries,
warnings about the usage of hidden APIs will be displayed in non-
final, non-user builds only.

Bug: 64382372
Test: make
Change-Id: Id0a4200f912ac3303026cb26b6d8974c47332828
diff --git a/core/main.mk b/core/main.mk
index 98f6ecb..38e9bd1 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -187,14 +187,16 @@
 
 #
 # -----------------------------------------------------------------
-# Enable dynamic linker developer warnings for userdebug, eng
-# and non-REL builds
+# Enable dynamic linker and hidden API developer warnings for
+# userdebug, eng and non-REL builds
 ifneq ($(TARGET_BUILD_VARIANT),user)
-  ADDITIONAL_BUILD_PROPERTIES += ro.bionic.ld.warning=1
+  ADDITIONAL_BUILD_PROPERTIES += ro.bionic.ld.warning=1 \
+                                 ro.art.hiddenapi.warning=1
 else
 # Enable it for user builds as long as they are not final.
 ifneq ($(PLATFORM_VERSION_CODENAME),REL)
-  ADDITIONAL_BUILD_PROPERTIES += ro.bionic.ld.warning=1
+  ADDITIONAL_BUILD_PROPERTIES += ro.bionic.ld.warning=1 \
+                                 ro.art.hiddenapi.warning=1
 endif
 endif