crash_dump: lower THREAD_COUNT in debuggerd_client.race for low-speed devices

The debuggerd_client.race tests the crash_dump process to finalize the
killed process within 2 seconds. The 2 seconds timeout for finalizing a
process, which has 1024 threads, is bit small for low-speed devices.
This CL lowers the bar in order to make such devices pass the test.
Wraping up 128 threads within 2 seconds looks safe.

Bug: 62600479
Test: debuggerd_test passes on low-speed devices.
Change-Id: I3089415961422e6933405d2c872913273425caff