Add the concept of apps to the battery stats parser.

AttributionKey is how we identify an app.  It contains either
a set of package names, read from the uid records from batterystats
(or later UidMap from statsd), or one of the hard coded SpecialApps.

Test: atest frameworks/base/tools/powermodel --host
Change-Id: If2dee6bffd2d3dafccfeff5c92bafc651b356b15