binder_rpc_fuzzer: introduce sleep

We don't have a good way now to wait for the server to finish processing
the incoming data. Sleep for a little bit, so that we can get more
deterministic results until we have a better way to do this.

Bug: 199324691
Test: binder_rpc_fuzzer
Change-Id: Ic1ba39061a08fc1d92f1931b2f1e6d0c895ac7e5