Implement Variant changes suggested by Manjunath Kudlur; simplify API.

1. Remove the self-mutating MaybeDecodeAndGet<>() in favor of the simple,
   original get<>().
2. Remove mutexes and locks from everything.

To allow tensorflow RPC to continue working transparently:

3. Add a registration mechanism for Variant decoding -- used by
   variant_coding within TensorFlow.
4. Add a MaybeDecodeAndCopy<>() const; for use by callers of Session::Run
   that get Tensors back which may contain encoded Variants.
5. Small bugfixes to Variant Encode impls and tensor FromProtoField and
   variant_coding.

PiperOrigin-RevId: 166383211