*** Reason for rollback ***

Retried -- fixed the offending part (some compilers require std::move() when returning unique_ptr which aren't of the exact returned type).

*** Original change description ***

Automated g4 rollback of changelist 214572852.

*** Reason for rollback ***

Breaks kokoro (not sure why I didn't see it before).

*** Original change description ***

Use the KeyFactoryBase in xchacha20_poly1305_key_manager.h.

Since there is no format proto, this also adds an "empty.proto" to tink.

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PiperOrigin-RevId: 214595273
GitOrigin-RevId: 66bfa9bf0fd5a4d3209064d15c7b991ec68692c1