libbinder: RPC allow RpcSession to be reusable

Previously, there were two ways setup could fail:
- very quickly (e.g. cannot create fd to poll)
- after some delay (e.g. second connectoin messed up)

In either case, 'false' is returned from setup* funtions. However, in
the second case, if setup* is called again, then it would result in an
abort. When connections fail for unrelated reasons, this sometimes
causes aborts in existing tests.

Two alternatives considered:
- switch to factory-type setup, this seems a bit heavy, chiefly because
  typically, only one RpcSession is needed, so it is annoying to have to
  create a factory and an object.
- disallow setup* from being called multiple times - this breaks some
  of our tests, and it adds work to clients.

Bug: 200737956
Test: manual
Change-Id: Ia6a69a7d2ca6c6835844cd9a90c7d24646a83526