Revert "Convert BuildIgnoreApexContritbutions variable to a boolean"

This revert was created by Android Culprit Assistant. The culprit was identified in the following culprit search session (http://go/aca-get/53721efb-c49e-4ce8-b96c-fd03598b4202).

Change-Id: Ifb82b8ec92b3ba8ad5da63ba497e4daad8093e4b
diff --git a/core/product.mk b/core/product.mk
index 39c3505..d64dde2 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -457,7 +457,7 @@
 
 _product_single_value_vars += PRODUCT_BUILD_FROM_SOURCE_STUB
 
-_product_single_value_vars += PRODUCT_BUILD_IGNORE_APEX_CONTRIBUTION_CONTENTS
+_product_list_vars += PRODUCT_BUILD_IGNORE_APEX_CONTRIBUTION_CONTENTS
 
 _product_single_value_vars += PRODUCT_HIDDEN_API_EXPORTABLE_STUBS