Improve the method names in BackoffParams

[shinfan]

Right now method names only fit for retry settings but not the timeout.

[garrettjonesgoogle]

I think we should just flatten the backoff params into retry params. Our client-side config will have them flattened, e.g.

      "initial_retry_delay_millis": 100,
      "retry_delay_multiplier": 1.2,
      "max_retry_delay_millis": 1000,
      "initial_rpc_timeout_millis": 2000,
      "rpc_timeout_multiplier": 1.5,
      "max_rpc_timeout_millis": 30000,
      "total_timeout_millis": 45000

[mziccard]

Closing this as was fixed in googleapis/gax-java#44