Don't back up Smooth Display if feature flag off

In the previous CL (ag/24838636), we decided to back up Smooth Display and Force Peak Refresh Rate even if the feature flag is off and then just convert the value in DisplayModeDirector based on the state of the feature flag. This was because it wasn't clear how to access the feature flag from the Settings module. This resulted in the feature partially working if the flag is off.

Bug: 313021502
Test: atest DisplayModeDirectorTest
Test: atest ForcePeakRefreshRatePreferenceControllerTest
Test: atest PeakRefreshRatePreferenceControllerTest
Test: atest SettingsBackupAgentTest
Test: atest SettingsBackupTest
Test: atest SettingsValidatorsTest
Change-Id: I3406bc5c5f49fe6102cdfe6934813a9c4073ac6f