libbinder: remove abstraction layer for get port

In preparation to move this code to another file. There were a few
bugs before:
- dead code
- getsockaddr error would lead to abort (but only inconsistent state in
  the old abstraction should have left for an abort)
- added check to ensure socklen_t when returned from getsockname is as
  expected

Bug: 185167543
Test: binderRpcTest
Change-Id: Ic79479b6a66226db309dfde134617af50c2365fe