Use multiarch builds for native samples

This pull request depends on the corresponding maven plugin pull request (https://github.com/jayway/maven-android-plugin/pull/230). Almost all native sample projects now include x86 compilation, and I've verified that armeabi/x86 library dependencies work for all but one project.

"mixed-java-native-with-apklib-dependency" does not handle the x86 dependency correctly, so I've left it unchanged for now.

**Please don't merge this until the maven plugin pull request has been accepted and released in a stable version.**