Android: Add versioning to file cache

The Qt files extracted by the app to the file system are never overwritten, so it's impossible to update the plugins and imports in an app that has been run once on the device already.

We need some versioning here. The easy solution would be to output the version code of the app to storage and update the file cache every time the app is updated, but this might cause more updates than necessary. A different approach would be to output the Qt version to storage and update whenever the app is built against a newer Qt version. This breaks if you are using modules which are updated on a different schedule than the rest of Qt.

I think the most flexible solution might be to support both and default to the latter.