RPC Binder: Update check in binderRpcTestTrusty

Fix assertion on numIncomingThreads being all zeroes
in Trusty in binderRpcTestTrusty, since Trusty does not
support client-side incoming connections due to lack
of threading.

Bug: N/A
Test: com.android.trusty.binderRpcTest
Change-Id: I3320d9928645a2ecf0d0c4d3ea83e78a8ba0fbc0