I'm developing an Android application and this is my first project, so I've also included 1 year free maintenance with the whole package so I want to know what should I include in the service, only UI changes and bug fixes or as well as adding a new feature will be included?
What to include as a service can depend on many factors that basically boil down to a compromise between the client's needs and your skills, time and financial situation.
In addition, one may consider whether the client is probably just a one-time sale or the possibility of an on-going relationship exists.
Generally, a service agreement is a way of attempting to stay on the client's radar, thus increasing the chances of more work.
You can ask for a fixed rate/month in which you tell your client that you will fix anything that breaks, so they don't have to worry.
You can also do library updates for security issues and similar.
Be very careful when including UI changes, as those could take a lot of time and effort to deliver and should be payed separately.