How to store accounting type data in Android?

I want to program an accounting app for Android, but I have no idea how to manage the storage of the data.

The data should be allowed to be stored wherever the user would like, e. g. in his Dropbox folder to sync with other devices.

Since I would like to get a working prototype as soon as possible, the storage system should be easily extendible.

I did look into persistent data storage in Android and I have a grasp on how I would store preferences, but SharedPreferences is no fit for this job.

However, this data seems to be suited well by an SQLite database (which I do not know how to use, yet). My problem is, that I do not know if you can store such database externally and if it is possible for (potential) apps on a computer to read it properly.

I followed a beginner tutorial that used JSON type data, but a fixed table as it seems to be in SQLite would suffice.

What is a good type of storage for my needs and how can I get started in learning how to use it?