Make @VisibleForTesting accessible for proguarding.

With only SOURCE retention, proguard can't be configured to keep
anything annotated as @VisibleForTesting, which is one of the major
reasons to use such an annotation in the first place - to prevent code
that's only used in unit tests from being stripped.

Change-Id: Ibe9a098236988001ba17a8c3e7fec52a73db501e
1 file changed