[Android] Change the default version to empty in packaging tool.

When the manifest.json version value is empty,
the webapp could be packed and installed successfully,
but shouldn't be show version in Apps.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1146
Merge pull request #1744 from junmin-zhu/defaultversion

[Android] Change the default version to empty in packaging tool.