Use screen on time as timebase for idling out apps
that have been inactive.
Store the time when an app was last active as an additional
package state in UsageStats. Compare it to screenOnTime to decide
if it's inactive.
Exclude device idle whitelist from apps that can go inactive.
Bug: 20066058
Change-Id: I709f9f31a9affa7ca6e1ae3e4c5729c5fb221669
06bf824628