Improve debug output when an ANR happens.

- Collect data at better times.
- Collect per-thread CPU usage as soon as possible after the ANR, and print
  in log.
- Based on new per-thread CPU usage, limit the number of processes we
  collect stacks from to not include inactive not interesting procs.
- Improve the way ProcessStats compute and reports its data.

Change-Id: I12b17fb47d593d175be69bb792c1f57179bf4fdf