Further work on issue #18640385: Add procstats test mode

Tune sampling periods:

- Lower minimum sample interval when in test mode.
- Add a check when sampling pss to make sure this is at least 1
  second since the pss changed, to reduce the amount of bad
  data (very small pss samples) that we see.

Change-Id: I624a7d0480f3f6b22d229744e028d13a2c7b0e89