Revert "Filter JUnit and android.test classes from applications using proguard"

This reverts commit f22aca1e.

Switching to use -systemjars instead of -libraryjars means that it is no longer
necessary to filter out the junit and android.test classes from jars to which
Proguard is being applied.

Bug: 30188076
Bug: 69156675
Test: make checkbuild
Change-Id: I36e14d28e01517927b458d69ade8ccb2c79a3340