libbinder: RPC fixup retry logic

By always reading from the pipe in the test, we can ensure we only try
to connect to a server once it's started. After this, additional retry
logic is unneeded.

Bug: 185167543
Test: binderRpcTest
Test: binderRpcTest (after introducing 100ms sleep before 'accept4' in
    RpcServer to simulate a server that is busy doing something else or
    busy accepting other calls from other clients)
Change-Id: I9b5b2876aa74e6ef1b6e8edc4c25124c29446b86