TOPIC:

Is building with Gradle in Android Studio supported now?

Is there a tutorial on this topic?

not yet, sorry.
We're still concentrated in developing an official xamarin interface on both iOS and Android.

We will include a KB article as soon as gradle support will be online.

Ok.
However, how do build projects including your library with Android Studio?

the demo project is AndroidStudio project.
so u can open it using AndroidStudio directly.
if u mean build your own APP, mostly u can following steps:
1. copy $(Demo)/ViewLib/src/main/jniLibs to $(Project)/src/main/jniLibs
2. copy $(Demo)/ViewLib/src/main/java to $(Project)/src/main/java
3. copy $(Demo)/viewLib/src/main/res to $(Project)/src/main/res
4. if u want using some UI codes: copy part or full of $(Demo)/PDFReader/src or $(Demo)/RDPDFReader/src to your projects.
wish to help u.

now, there is guide document, see link:
guide.pdf