Persist the db key to storage

save_to_prefs.kt

/**
 * Save the storable instance to preferences.
 *
 * @param storable a storable instance
 */
fun saveToPrefs(context: Context, storable: Storable) {
    val serialized = Gson().toJson(storable)
    val prefs = context.getSharedPreferences("database",
        Context.MODE_PRIVATE)
    prefs.edit().putString("key", serialized).apply()
}