[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 tool also can remove, list and search local extensions.

To package these extensions into app, you don't have to provide full path.
Developer just need provide the name of extension, such as "iap", to the
--extension option of make_apk.py.

BUG=XWALK-1947