binderThroughputTest: skip benchmark loop for servers

Skip the entire benchmarking loop if this is a server worker instead of
asking the same fixed question on every single iteration.

Change-Id: I66d1578132099ea1cb4a3b5a10a830bb11b579a2
Test: Ran a benchmark on cuttlefish, and it did not get stuck
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>