Mark Soong-exported make variables as readonly

Using new .KATI_READONLY extension.

Test: build/tooks/kati_all_products.sh on AOSP master and internal
Change-Id: I71794c1268456c3403a351c1b242f1e78b060355
diff --git a/android/makevars.go b/android/makevars.go
index 149f2f8..e431f11 100644
--- a/android/makevars.go
+++ b/android/makevars.go
@@ -170,6 +170,7 @@
 else
   $(1) := $$(SOONG_$(1))
 endif
+.KATI_READONLY := $(1) SOONG_$(1)
 endef
 
 my_check_failed := false