Greendao tasks should declare inputs and outputs

In order to enable up-to-date checks in Gradle. The greendao Gradle task should define inputs and outputs.

This will speed up builds.

Here is some more information on how to declare inputs and outputs in the Gradle task. https://docs.gradle.org/current/userguide/more_about_tasks.html#sec:up_to_date_checks

If the Gradle plugin sources were available somewhere, I would love to contribute this feature.