Dynamically determine protobuf version in build.gradle. #1757
Description
For self-service clients, e.g., google/maps/fleetengine, use gradle to build the client library. The protobuf version is defined in gax-java, which is not synced with protobuf version defined in googleapis.
This result in build failures when the protobuf version in googleapis is higher than version in gax-java (code generated by higher version of proto compiler may not compatible with lower version protobuf runtime environment).
Therefore, the protobuf version in gax-java should be synced with protobuf version in googleapis, rather than hard coded in a property file.
See b/284292352 for more info.