[TF] Variant improvements.

1. Change Variant Decode to accept VariantTensorData (non-ref).

This should allow some optimization in the future.

In the meantime it means removing the variant.h include from tensor.h, since
variant_encode_decode.h now relies on tensor.h and variant.h now relies on that.

It also means we found a bunch of places where tensor.proto.h, variant.h, and
mutex.h were being imported through tensor.h (along with a bunch of other crap);
so now we directly import them in order to compile.

2. Move Variant registry to use TypeIndex instead of a TypeName string; this should
speed up registry lookups.

PiperOrigin-RevId: 212478896