[Android] Packaging enhancement for external extensions

Add extension_manager.py to download online extension repo.
Usage of extension_manager.py:

1. Add extension
$ python extension_manager.py \
--add=https://github.com/crosswalk-project/crosswalk-android-extensions.git

2. List all extensions
$ python extension_manager.py -l

3. Search extension
$ python extension_manager.py --search=crosswalk*

4. Remove extension repo
$ python extension_manager.py --remove=crosswalk-android-extensions

5. Enable extension
$ python extension_manager.py --enable=crosswalk*

6. Disable extension
$ python 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
Merge pull request #2283 from shawngao5/extension_manager

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