The core Java libraries are part of the Android runtime
(see image below) layer, but from developer point of view it doesn't matter. What you can do and cannot do is tied to Android SDK level you use in your project.

Which android:minSdkVersion
you should use?
You can check the Android documentation. For every instruction /
command / method / properties, at the top right you'll find the API
level at which you are able to access said property. Clicking on the
API level will take you to a page which contains a table that
translates API level to Android versions.
As a general principle set android:minSdkVersion
as low as possible. You will get a compiler error when you use something not supported by that level. This way you can support as many users as you can. See this for details.
Here is a recommended reading on this subject.