refactor Bazel variable export

Most of the variable export code for cc modules can be re-used for
exporting variables for java modules. Refactor this code into a more
composable structure for reuse.

Test: build/bazel/bp2build.sh
Test: manual comparison of
  out/soong/soong_injection/cc_toolchain/constants.bzl
  with previous output
Change-Id: Ie5a6fee08cc888b7dc69c3e324e5c3f8aa269a8f
diff --git a/android/Android.bp b/android/Android.bp
index c072ac2..49d5b91 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -36,6 +36,7 @@
         "bazel_handler.go",
         "bazel_paths.go",
         "config.go",
+        "config_bp2build.go",
         "csuite_config.go",
         "deapexer.go",
         "defaults.go",
@@ -96,6 +97,7 @@
         "bazel_handler_test.go",
         "bazel_test.go",
         "config_test.go",
+        "config_bp2build_test.go",
         "csuite_config_test.go",
         "defaults_test.go",
         "depset_test.go",