libbinder: allow multiple outgoing threads for single-threaded

Additional outgoing threads in a single-threaded client do not
actually require more threads, so they should be safe to enable.
We check the number of incoming threads per RpcSession instead,
since those create actual OS threads.

Prevents a TOCTTOU issue between calls to setup*Client() and
setMaxIncomingThreads() by adding a new mStartedSetup variable
that is set early during setupClient, and any calls to RpcSession
setters are fatal failures after that point.

Bug: 224644083
Test: atest binderRpcTest*
Change-Id: Id0ce2cda63e781ecfba86180f3c523be9044d408