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

Add --get-online-extensions and --online-extensions-path to package
external extensions.

Usage:
Download and build extensions:
make_apk.py \
--get-online-extensions=https://github.com/crosswalk-project/crosswalk-android-extensions.git \
--online-extensions-path=/home/xgao/crosswalk-src/crosswalk-android-extensions/

Build with extension iap:
python make_apk.py --package=org.intel.iap \
--manifest=/home/xgao/crosswalk-src/crosswalk-android-extensions/examples/iap/iap.json \
--extensions=iap \
--online-extensions-path=/home/xgao/crosswalk-src/crosswalk-android-extensions/ \
--arch=x86

Default path of --online-extensions-path is 'extensions' with the same path level of make_apk.py.

BUG=XWALK-1947