Fix issue with DiscreteValuedDistribution that caused issues when snapshotting it.
Fix issue with DiscreteValuedDistribution that caused issues when snapshotting it.

PiperOrigin-RevId: 338505836
Change-Id: I869d244a5d449fc5ddb6e1389362a632e4d9912d

deepmind/acme,deepmind/acme