Idle timebase

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