libbinder: RPC prevent oneway cmd buildup

Previously, binderRpcTest OnewayStressTest was having a flaky hang @ 100
calls because the sockets were filling up with decref counts (these are
currently a bit long because they contain RpcAddress - these may need to
be smaller - see b/182939933). I temporarily reduced this to 50 calls to
avoid the flake, but this is the real fix (which also increases the
number of calls to 500).

Now, when we send a oneway call, we also drain any refcounts ("CONTROL")
commands. We disallow processing of any transactions in this case (this
would indicate a misbehaving client).

Fixes: 189272263
Test: binderRpcTest
Change-Id: If92af65f5e5a920e39046af6b10f675b5c26cd6e