[Android] Replace external extensions with source instead of jar

The prebuild jar files of external extensions may out of date as
the runtime is walking forward. So place them with source code.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2539
you need to add `xwalk_external_extensions` to dependency of xwalk_builder, and i suggest to rename it to `xwalk_packaging_tool_test`