Make apps active when charging.

This undoes part of the work to remove parole windows from
AppStandbyController. In the past, UsageStatsManager.isAppInactive would
return false when the device was charging. This wasn't documented
behavior, but was intentional, so removing it was the wrong thing to do.
I've updated the documentation to explicitly call out the behavior and
added it back so that the behavior is consistent with previous versions.

Bug: 148988544
Test: atest CtsUsageStatsTestCases:UsageStatsTest
Test: atest AppIdleHistoryTests
Test: atest AppStandbyControllerTests
Change-Id: If5cad80b693a1e043ae64b8206e28abb60f83853