Fix false alarm on thread suspend timeout

Root cause:
  CLOCK_REALTIME will jump backward/forward when system time-of-day clock is changed.
  It implies now_abs_ts will jump a lot across end_abs_ts.
  Then, it makes a false alarm (process crash) on thread suspend timeout when doing ComputeRelativeTimeSpec.

      if (ComputeRelativeTimeSpec(&rel_ts, end_abs_ts, now_abs_ts)) {
        return false;  // Timed out.
      }

Solution:
  Use CLOCK_MONOTONIC instead

Change-Id: I768af52b05ee1548bb291f7d5e2f389ec85e0e71