Refactor isAppHibernationEnabled check in AppHibernationService

Directly return the static field to avoid querying DeviceConfig
everytime.

Bug: 181183614
Test: atest AppHibernationServiceTest
Test: atest CtsWindowManagerDeviceTestCases:ActivityMetricsLoggerTests
Test: atest CtsStatsdAtomHostTestCases:AppStartStatsTests

Change-Id: Id01a701dadb76655a24effbd9d3ce011d7f2f686
3 files changed