Add optional features to an Android App using add-on apk

Add optional features to an Android App using add-on apk

I have an Android App that collects and stores data entered by the user.
Right now, I don't use the internet permission to give confidence to the
users that their data is secure in the app. However, some users have
requested me to include a feature to sync data between multiple devices
they use to access the app.
To ensure I take the data confidentiality considerations of my users, I
want to add this sync feature as an optional feature preferably thru an
add-on app so that I can device to charge for it separately.
I have looked at multiple options of doing this but have not been able to
zero in on the approach. I am thinking of something like this:
First App -> Checks if second app is installed (which means the user has
purchased the sync service -> Now the first app invokes a service in the
second app -> Second app has internet permission and can access data of
the first app -> Second app syncs the data with the web server
The same way together the first app and second app will check if there is
updated data available on the server and copy it to the local device.
I have the following questions.
1) How can I access the database of the first app from the second app?
2) How can I invoke a service in the second app from the first app?
3) How can I invoke a service/activity in the first app from the second app?