Convert system forced reasons to flags.

Switching the reasons to flags allows us to track mulitple reasons for
an app being in a bucket.

Bug: 149507105
Test: atest FrameworksServicesTests:AppIdleHistoryTests
Test: atest FrameworksServicesTests:AppStandbyControllerTests
Change-Id: I6fb9e980652ac76e927fbba3d65da2b39b55f1b9
6 files changed