binderThroughputTest: only parse user latency once

The latency seems already capped by atoi() so there is no need of doing
a reparse via strtoull(). Besides any values above INT_MAX microseconds
don't really make sense for this test. The conversion into nanoseconds
doesn't overflow this way either: INT_MAX * 1000ull < UINT64_MAX.

Change-Id: I603bbeee2e94c9975e502c1af1c1c2bbcc82f79a
Signed-off-by: Carlos Llamas <cmllamas@google.com>