[google-auth-library-java] ServiceAccountCredentials with SSJ flow will get error when invoking refreshAccessToken()

[refreshAccessToke()](https://github.com/googleapis/google-auth-library-java/blob/89201558db913d9a71b3acccbab8eb0045ada6de/oauth2_http/java/com/google/auth/oauth2/ServiceAccountCredentials.java#L496) posts request to OAuth2 token server (see [code](https://github.com/googleapis/google-auth-library-java/blob/89201558db913d9a71b3acccbab8eb0045ada6de/oauth2_http/java/com/google/auth/oauth2/ServiceAccountCredentials.java#L507)).
This code path is not exercised for TPC flow via getRequestMetadata(), but it is an public method and is possible to accidentally get triggered by customer's code.

One invocation within this library (that we are patching in googleapis/google-auth-library-java#1528 ) is [here](https://github.com/googleapis/google-auth-library-java/blob/89201558db913d9a71b3acccbab8eb0045ada6de/oauth2_http/java/com/google/auth/oauth2/ImpersonatedCredentials.java#L493)