libbinder: single-threaded binderRpcTest cleanup

Removes BinderRpcThreads test suite and moves each
single-threaded check to every individual test to allow us
finer-grained control over which tests run in which threading
configuration. Adds a helper clientOrServerSingleThreaded()
method to BinderRpc.

Bug: 230135749
Test: atest binderRpcTest binderRpcTestSingleThreaded
Change-Id: Ie598c39288a982e1cc1403b506c2cb2bf970b882