RPC Binder: flush refcounts more reliably

This:
- always flushes refcounts even if there is an error.
  This helps avoid a leak in some cases.
- if there is a protocol error while flushing refcounts,
  this does not try to write it back, but just returns.
  This is to limit fallout.

Bug: 419074122
Bug: 419364025
Test: binderRpcTest
Change-Id: I669cb264645fa4b2f36e47aa5af89fc69bc1cad6