[Android] Packaging enhancement for external extensions

Add extension_manager.py to download online extension repo.
Usage:
extension_manager.py
--add=https://github.com/shawngao5/crosswalk-android-extensions.git
--name=iap --project=/home/xgao/tmp/iap/
Downloaded extensions are saved under <project>/extensions/

This tools also can remove, list and search local extensions.

To package these extensions into app, full path is not neened. Developer
just provides the name of extension, such as "iap", to the --extension option
of make_apk.py.

BUG=XWALK-1947