aconfig: add default-permission argument for create-cache command

Aconfig set flag default permission as READ_WRITE regardless of the
release configuration. This change enables the caller of create-cache to
set the default permission for all flags, if the flag doesn't have a
value.

Test: atest aconfig.test
Bug: 294417368
Change-Id: I1ba19e1ba793cd2ae59923b136b9b50a92315ece
diff --git a/core/soong_config.mk b/core/soong_config.mk
index bd6cfbb..3704987 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -320,6 +320,7 @@
 
 $(call add_json_str, ReleaseVersion,    $(_RELEASE_VERSION))
 $(call add_json_list, ReleaseAconfigValueSets,    $(RELEASE_ACONFIG_VALUE_SETS))
+$(call add_json_str, ReleaseAconfigFlagDefaultPermission,    $(RELEASE_ACONFIG_FLAG_DEFAULT_PERMISSION))
 
 $(call add_json_bool, KeepVndk, $(filter true,$(KEEP_VNDK)))