[Android] Add new command for make_apk.py to package external extensions 

Add extension_manager.py to download online extension repo.
Usage of extension_manager.py:

1. Add extension
$ ./extension_manager.py \
--add=https://github.com/crosswalk-project/crosswalk-android-extensions.git

2. List all extensions
$ ./extension_manager.py -l

3. Search extension
$ ./extension_manager.py --search=crosswalk*

4. Remove extension repo
$ ./extension_manager.py --remove=crosswalk-android-extensions

5. Enable extension
$ ./extension_manager.py --enable=crosswalk*

6. Disable extension
$ ./extension_manager.py --disable=crosswalk*

Usage of make_apk.py
$ python make_apk.py --package=your.app.name --manifest=\<your_manifest_path\>

BUG=XWALK-1947
1) ok, so `enable` is true by default, then you can use `if not data.get('enable', True):`, `dict.get(key, default)` will return default if key doesn't exist.
2) If here, you have the extensions_path and extension_name, i suggest you to use GetExtensionStatus(). Because if in future, if you decide to change they way to get extension status. (e.g. change 'enable' to be by default False if not set), you won't forget here to change.
Avoid duplicated logic is always good.