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/apex/Android.bp b/apex/Android.bp
index 480ad06..018d030 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -26,6 +26,7 @@
         "apex_sdk_member.go",
         "apex_singleton.go",
         "builder.go",
+        "bp2build.go",
         "deapexer.go",
         "key.go",
         "prebuilt.go",