Throttle jobs for idle apps

First pass at delaying jobs from apps that are idle.

TODO: Throttle syncs
TODO: Provide a periodic point at which apps are checked for idleness.

Apps that switch to foreground process state are tracked by UsageStats
as an INTERACTION event that affects the last-used timestamp.

JobScheduler's logic for when an app is ready is trumped by the idleness
of the app, and only if the battery is not charging. When charging state
changes, we update the idle state of all the tracked jobs.

android package is whitelisted.

Bug: 20066058
Change-Id: I0a0acb517b100a5c7b11e3f435f4141375f3451f