[sdk-platform-java] The order of entries in generated `gapic_metadata.json` and `package-info.java` should be deterministic

Currently, the order entries in `gapic_metadata.json` and `package-info.java` is the same as the corresponding proto files in `BUILD.bazel`.

However, the proto files can be reordered manually which leads to undeterministic output in `gapic_metadata.json` and `package-info.java`.

The entries should be in a deterministic order (e.g., alphabetic order), regardless the order of protos.