binder: RpcTransport: change to size_t.

- Change return type to Result<size_t> because negative numbers
  (-1) are converted to an Error().
- Change size argument from int to size_t because size_t is unsigned
  and is the best type to represent a "size".

Test: pass
Bug: 190868302
Fixes: 195592175
Change-Id: Ide26d2d5cc11a9b776fc3dc0a2281300f611b327