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

bp2build/conversion_test.go

@@ -103,6 +103,14 @@ func TestCreateBazelFiles_Bp2Build_CreatesDefaultFiles(t *testing.T) {
 			dir:      "java_toolchain",
 			basename: "constants.bzl",
 		},
+		{
+			dir:      "apex_toolchain",
+			basename: GeneratedBuildFileName,
+		},
+		{
+			dir:      "apex_toolchain",
+			basename: "constants.bzl",
+		},
 		{
 			dir:      "metrics",
 			basename: "converted_modules.txt",