Adds default public visibility to the Cirq V2 protos. 
Adds default public visibility to the Cirq V2 protos. 

This is so external libraries (e.g., TFQuantum) can depend on them when depending on Cirq via a remote archive in blaze. (#1683)
quantumlib/Cirq,balopat/Cirq,balopat/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,quantumlib/Cirq