[Android] Integrate external extensions in make_apk.py

The API for external extensions are basically ready so this is to
enable the packaging tool to package external extensions into web
app APKs.

To add one extension to be packged, developers need to do below steps:
1. Create a directory named 'foobar' and add 3 files called 'foobar.jar'
'foobar.js' and 'foobar.json' into the directory.
2. Package it when running make_apk.py:
   python make_apk.py --extensions="/path/to/foobar"

BUG=#558