libbinder: RPC read connection info on 2nd thread

Don't allow the main server thread to be blocked when reading a new ID.
Gotta be available for reading new clients.

Future consideration will be timeouts for reads/writes or trying to
detect DOS here in general, but this CL is what is needed for the
fuzzer.

Bug: 185167543
Bug: 182938024
Test: binderRpcTest
Change-Id: I8ba4a6ce6d1d07f3c36ac554ccb6cc403a15db2b