Replace @NeededForTesting w/ proguard keep flags (1/2)

* Replace -keep of entire c.a.c.common package
  with individual classes
* Remove all use of @NeededForTesting annotations
  (and the file itself since @VisibleForTesting
  accomplishes the same thing)

Test: Ran user build ContactsTests on Nexus 5 MOB30H

Bug: 30976481

Change-Id: I8d9a4d6b3409a159a108b39c1c4837a19e2325ca