Default BUILD_BROKEN_ANDROIDMK_EXPORTS to false

See https://android.googlesource.com/platform/build/+/master/Changes.md#export_keyword

Bug: 73959648
Test: build/soong/build_test.bash on downstream branches
Change-Id: I120e0cc381bae71101ec9ca48ee7cbe8a0b9615f
diff --git a/core/envsetup.mk b/core/envsetup.mk
index c7c3f92..b92b485 100644
--- a/core/envsetup.mk
+++ b/core/envsetup.mk
@@ -302,8 +302,7 @@
 
 CHANGES_URL := https://android.googlesource.com/platform/build/+/master/Changes.md
 
-# "" is equivalent to true currently.
-ifeq ($(BUILD_BROKEN_ANDROIDMK_EXPORTS),false)
+ifneq ($(BUILD_BROKEN_ANDROIDMK_EXPORTS),true)
 $(KATI_obsolete_export It is a global setting. See $(CHANGES_URL)#export_keyword)
 endif