how to reference an asset in a library project

This answer is out of date, the gradle build system and AAR files support assets.

From the Android Docs:

Library projects cannot include raw assets

The tools do not support the use of raw asset files (saved in the assets/ directory) in a library project. Any asset resources used by an application must be stored in the assets/ directory of the application project itself. However, resource files saved in the res/ directory are supported.

If you want to include files from a Library project, you'll need to put it in the resources instead of the assets. If you're trying to load HTML files from your library project into a WebView, this means that you'll have to go a more roundabout method than the usual asset URL. Instead you'll have to read the resource data and use something like loadData.

This is now possible using the Gradle build system.

Testing with Android Studio 0.5.0 and v0.9 of the Android Gradle plugin, I've found that files such as

MyLibProject/src/main/assets/test.html

are correctly packaged in the final application and can be accessed at runtime via the expected URL:

file:///android_asset/test.html