To appease PMD, we now have a three rulesets in `config/pmd/*.xml`:
* `rules.xml`: The bulk of the rules, used by both main and test code.
* `rules-main.xml`: Rules specific to the andoid client code.
* `rules-test.xml`: Rules specific to test code.
The rationale is because checkstyle by default checks for "too many static
imports", which is a fair call. However in JUnit4 code, it is common to
import many `assert*` static methods.
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