Use class retention for VisibleForTesting annotations

The VisibleForTesting annotations are used by proguard to prevent
stripping code that is needed at runtime for tests.  The annotations
were marked with source retention, which worked fine with Jack's
proguard implementation.  When we switched back to javac + proguard,
the VisibleForTesting annotations are gone by the time proguard
sees the classes, so the annotated classes and methods can still
be stripped.

Move the VisibleForTesting annotation to class retention, which
matches guava's equivalent annotation.

Bug: 64365384
Test: m -j MtpDocumentsProvider &&
      javap -cp out/target/common/obj/APPS/MtpDocumentsProvider_intermediates/classes-proguard.jar com.android.mtp.MtpRoot

Change-Id: I63a467c2f0a21383eb9e4c9563fd6de2d994a373