Output build_apk_secondary_abi to build_vars.txt

Change-Id: Id4153b6fdd76aff58f682c92e5e67614cf30bfa4
Reviewed-on: https://chromium-review.googlesource.com/1205609Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Ben Mason <benmason@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588861}

build/android/BUILD.gn

@@ -60,6 +60,9 @@ if (enable_java_templates) {
     _data += "android_secondary_abi_toolchain=" +
              rebase_path(_secondary_label_info, root_build_dir) + CR
   }
+  if (defined(build_apk_secondary_abi)) {
+    _data += "build_apk_secondary_abi=$build_apk_secondary_abi" + CR
+  }
   write_file(android_build_vars, _data)
 }