Move setAppStandby logic inside AppStandbyController.

Since we've moved AppStandbyController inside the JobScheduler mainline
module boundary, we need to move the logic of setAppStandby over so that
it's consistent for callers and easy to do the right thing.

Bug: 137763703
Bug: 145551233
Test: atest AppIdleHistoryTests
Test: atest AppStandbyControllerTests
Test: atest UsageStatsTest
Change-Id: Ie2ccb0508c3a3887ab3debc47e78427477d2d77b