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

android/api_levels.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"strconv"
 
+	"android/soong/bazel"
 	"android/soong/starlark_fmt"
 )
 
@@ -393,10 +394,10 @@ func printApiLevelsStarlarkDict(config Config) string {
 }
 
 func StarlarkApiLevelConfigs(config Config) string {
-	return fmt.Sprintf(`# GENERATED FOR BAZEL FROM SOONG. DO NOT EDIT.
+	return fmt.Sprintf(bazel.GeneratedBazelFileWarning+`
 _api_levels = %s
 
 api_levels = _api_levels
 `, printApiLevelsStarlarkDict(config),
 	)
-}
+}