[IFRT] Migrate `MemoryKind` use to use `value()` and `is_default()`

Migrate `xla::ifrt::MemoryKind` uses to use the new `value()` and `is_default()` semantic methods instead of `*memory_kind()` and `!memory_kind().has_value()` methods. The latter form has been deprecated because all memory kinds are always canonicalized at construction time, and there will be no case where `memory_kind().has_value() == false`.

PiperOrigin-RevId: 971988610