Change FileDescriptorSet to be an optional field in self describing
message proto file to accomodate any future plans to send schemas
separately from the protobuf messages.

Change-Id: I4a2b96585b20102a850503cc23ed0a354bbb2771
Merge "Change FileDescriptorSet to be an optional field in self describing message proto file to accomodate any future plans to send schemas separately from the protobuf messages."