In my (small) personal experience, save your game data with SharedPreferences. Android will do all the *dirty* work for you, and it is fast.
Also, I made a quick search, and you can share same app data if your second app is a child.
free version: com.package.game
paid version: com.package.game.paid
You must use paid as a child of free (not otherwise). Because if you use:
free version: com.package.game.free
paid version: com.package.game
This will not detect your data (at least that is what I think), because your original package don't have any child.
For more information, read matt5784 answer HERE
. (Just the first part, you don't need to backup nothing, etc):
it appears to be possible to share preferences between applications if a few requirements are met. First, if you want App B to be able to access App A's preferences the package name of App B must be a child of App A's package name (e.g. App A: com.example.pkg App B: com.example.pkg.stuff). Additionally, they can't be wanting to access the file at the same time