Filter JUnit and android.test classes from applications using proguard

The new javac/desugar/proguard/dx based toolchain can fail if
there are duplicate classes in the -injars and -libraryjars
passed to proguard. That causes problems for b/30188076 which
is attempting to remove various junit and android.test classes
from the API because it requires the changes to add those
classes statically to applications are submitted simultaneously
with the change to remove the classes from the API which is not
feasible.

This change simply causes Proguard to ignore the classes from
the application JAR so it will always use them from the library.
That allows the changes to be done separately and only requires
that this change is reverted simultaneously with the change to
update the API.

Bug: 30188076
Test: make checkbuild and make -j ANDROID_FORCE_JACK_ENABLED=disabled checkbuild
Change-Id: I6ed6c45a159d6261d90245551aa2913cc82d2e8b