Add base sdk extension version to the config

So that it can be used by prebuilt_apis in follow-up CLs.

Bug: 220086085
Test: m nothing && inspect soong.variables
Change-Id: If987b8aef9802c52a751ea5351cab2a3df6f2e00
diff --git a/android/variable.go b/android/variable.go
index ff77fef..dbe7b04 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -191,6 +191,7 @@
 	Platform_sdk_version_or_codename          *string  `json:",omitempty"`
 	Platform_sdk_final                        *bool    `json:",omitempty"`
 	Platform_sdk_extension_version            *int     `json:",omitempty"`
+	Platform_base_sdk_extension_version       *int     `json:",omitempty"`
 	Platform_version_active_codenames         []string `json:",omitempty"`
 	Platform_vndk_version                     *string  `json:",omitempty"`
 	Platform_systemsdk_versions               []string `json:",omitempty"`