Move serialized dictionary feature behind the flag.

BROTLI_SHARED_DICTIONARY_SERIALIZED enum value is a part of API,
but it should not be used (will cause failures).
Changing how serialized dictionaries work won't be considered as an API change, until this feature is enabled.
Enabling this feature in the future will be considered as a "compatible" change.

PiperOrigin-RevId: 558091676