Reduce widget exemption status.

Apps with active widgets should be fine operating in the ACTIVE bucket
instead of being fully exempted from app standby. They were previously
put in the EXEMPTED bucket because we previously didn't have a concept
of exemption levels.

Bug: 156236665
Test: atest FrameworksServicesTests:AppIdleHistoryTests
Test: atest FrameworksServicesTests:AppStandbyControllerTests
Change-Id: Iea30f00ca8199f487bcc3281e4ae3f98b41d369f