libbinder: RPC sock addr associated with RpcServer

This is in preparation for RpcServer having a single thread which loops
w/ accept4. With this CL, since this is moved, there is no way to
identify which thread comes with which client, so the ability to have
multiple clients is temporarily limited (fixed in CL above this). Either
way, nothing currently needs this functionality.

Bug: 185167543
Test: binderRpcTest
Change-Id: I48821970f7cbcb3fec0df00465296072d96db608