I'm looking for a simple solution for what I think is an easy problem!
I have an android app that deals a lot with the camera and reads NFC tags, and I want to have a common database shared among all the instances of the app. I've created a simple diagram to explain this:

So far I've been considering to use Google App Engine to create a free Google Cloud Datastore or even Google Cloud SQL. And it seems the best way to deal with it is using Google App Engine Endpoints to work with Android:
https://developers.google.com/appengine/docs/java/endpoints/consume_android
following this tutorial:
https://developers.google.com/eclipse/docs/endpoints-androidconnected-gae
I just want to go for the easiest solution. The database will have some small photos and data, but not big deal! The operations will be also quite simple.
Thanks for your opinion!!!
If you want the simplest solution then you probably want one of the Backend as a Service options.
In particular, Google has now entered this field with their 'mobile backend starter': https://developers.google.com/cloud/samples/mbs/
One thing that is very nice about using Google's option to back an Android app is that it is built on GAE/J so it can easily be extended using Java.