Separate active codenames and preview codenames.

Using PLATFORM_VERSION_ALL_CODENAMES (which is a bad name for
historical reasons) for discovering preview API levels doesn't work in
the narrow window where the branch's development is not for the latest
release. Create a second variable for preview codenames and expose it
to soong.

Bug: None
Test: None
Change-Id: I8c271c1d646ea8285553614c6bca944227a0e643
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 0101796..1dd02d0 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -31,6 +31,7 @@
 $(call add_json_val,  Platform_sdk_extension_version,    $(PLATFORM_SDK_EXTENSION_VERSION))
 $(call add_json_val,  Platform_base_sdk_extension_version, $(PLATFORM_BASE_SDK_EXTENSION_VERSION))
 $(call add_json_csv,  Platform_version_active_codenames, $(PLATFORM_VERSION_ALL_CODENAMES))
+$(call add_json_csv,  Platform_version_all_preview_codenames, $(PLATFORM_VERSION_ALL_PREVIEW_CODENAMES))
 $(call add_json_str,  Platform_security_patch,           $(PLATFORM_SECURITY_PATCH))
 $(call add_json_str,  Platform_preview_sdk_version,      $(PLATFORM_PREVIEW_SDK_VERSION))
 $(call add_json_str,  Platform_base_os,                  $(PLATFORM_BASE_OS))