Code review comment for lp:~mir-team/mir/android-input-transport-scatter-send-event

OK codewise, but note this is a protocol break that isn't a client ABI bump.

Before release we need to update mir::protobuf::oldest_compatible_protocol_version() and possibly mir::protobuf::current_protocol_version() (the latter only if we don't add any client API functions and bump MIR_CLIENT_MINOR_VERSION).

mir::protobuf::oldest_compatible_protocol_version() should return protocol_version(MIR_CLIENT_MAJOR_VERSION, 3);

Otherwise we'll accept connections from older libmirclient binaries but send input events they cannot parse and (if we don't bump MIR_CLIENT_MINOR_VERSION) successfully connect new clients to 0.18 servers which do not supply the correct events.

We can either make these changes here, or accept a promise elsewhere but we need to be sure of the approach before landing this branch.