Migrate AppStandbyController to DeviceConfig.

This migrates flags inside AppStandbyController to DeviceConfig.
DeviceConfig is the recommended way and using it means
that we only operate/parse the set of flags that changed instead of
every single flag whenever one of them changes.

Bug: 124466289
Test: atest AppIdleHostTest
Test: atest CtsUsageStatsTestCases:UsageStatsTest
Test: atest FrameworksServicesTests:AppIdleHistoryTests
Test: atest FrameworksServicesTests:AppStandbyControllerTests
Change-Id: I1aa5afc7f96e121bceccce44def4b86883b0a3c9