It is not possible to export an Android library as a jar file. It is possible, however, to export it as aar file. Aar files being the new binary format for Android libraries. There’s info about them in Google I/O, the New Build System video.
First, build the library in Android Studio or from command line issuing gradle build from your library’s root directory.
This will result in <yourlibroot>/libs/build/yourlib.aar file.
This aar file is a binary representation of your library and can be added to your project instead of the library as a dependency project.
To add aar file as a dependency you have to publish it to the maven central or to your local maven repository, and then refer the aar file in your project’s gradle.build file.
However, this step is a bit convoluted. I’ve found a good explanation how to do so here:
http://www.flexlabs.org/2013/06/using-local-aar-android-library-packages-in-gradle-builds