Fix and improve the low pressure filter.

Separate the tracking id assignment from low pressure filter. Ignore
the original tracking id from kernel driver, and assign it based on
previous HardwareState.

BUG=chromium-os:29323
TEST=unittest; should make no impact to users.

Change-Id: Ia295e6534fe91475140fdb4cb301b031ad8f82d2
Reviewed-on: https://gerrit.chromium.org/gerrit/21013
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Commit-Ready: Chung-yih Wang <cywang@chromium.org>
Reviewed-by: Chung-yih Wang <cywang@chromium.org>
Tested-by: Chung-yih Wang <cywang@chromium.org>