Export default apex version from updatable_modules.go to Bazel's
apex_toolchain through soong_injection.

This is necessary to keep both Bazel and Soong in sync.

Test: presubmit
Bug: 238723069
Fixes: 238153998

Change-Id: Ibf4a8ed91d358dc7800c10e4112485a52f0606bb
diff --git a/bp2build/conversion.go b/bp2build/conversion.go
index 4246f7d..8cf9bea 100644
--- a/bp2build/conversion.go
+++ b/bp2build/conversion.go
@@ -10,6 +10,8 @@
 	cc_config "android/soong/cc/config"
 	java_config "android/soong/java/config"
 
+	"android/soong/apex"
+
 	"github.com/google/blueprint/proptools"
 )
 
@@ -28,6 +30,9 @@
 	files = append(files, newFile("java_toolchain", GeneratedBuildFileName, "")) // Creates a //java_toolchain package.
 	files = append(files, newFile("java_toolchain", "constants.bzl", java_config.BazelJavaToolchainVars(cfg)))
 
+	files = append(files, newFile("apex_toolchain", GeneratedBuildFileName, "")) // Creates a //apex_toolchain package.
+	files = append(files, newFile("apex_toolchain", "constants.bzl", apex.BazelApexToolchainVars()))
+
 	files = append(files, newFile("metrics", "converted_modules.txt", strings.Join(metrics.convertedModules, "\n")))
 
 	files = append(files, newFile("product_config", "soong_config_variables.bzl", cfg.Bp2buildSoongConfigDefinitions.String()))