Adjust activity manager process OOM adj.

Modify OOM adj classes a bit, to take into account the new
heavy weight app type, and give "foreground services" their
own category to have a bettery chance to manager them when
things go wrong.

Also add some new code to battery stats to keep a history
of changes to the battery level.

Change-Id: I29f5ab6938777e1a7eafd7d8c38b5e564cc9f96a