Implement correct move constructor and assignment for absl::strings_internal::OStringStream, and mark that class final.

This should be explicit per https://google.github.io/styleguide/cppguide.html#Copyable_Movable_Types, and is currently hard to infer due to multiple inheritance.

PiperOrigin-RevId: 474311032
Change-Id: I72d7adcb9f7a991c19c26bc7083a4df31de5da49