[jnigen] Support Maven version 4 (https://github.com/dart-lang/jnigen/issues/261)

From version 4 Maven throws an error if we use descriptorId as we
should be using descriptorRef instead.
descriptorRef also works with Maven 3.

To enable support for Maven 4 we have changed the Maven command used
to build ApiSummarizer.jar from 'mvn assembly:assembly` to
`mvn compile`.
Jnigen/java's README contains instructions for manually building
ApiSummarizer.jar. These instructions need to be updated to use
the new maven commands 'mvn compile' and 'mvn test'.