Add a feature flag for the special access to the long background task

The entry should be hidden if the RUN_LONG_JOBS can't be toggled.

Bug: 255821578
Test: atest AppFilterRegistryTest
Test: make -j RunSettingsRoboTests \
ROBOTTEST_FILTER="LongBackgroundTasksDetailsTest|
LongBackgroundTasksDetailsPreferenceControllerTest"
Test: Manually check the Settings page

Change-Id: Ib1c58d93b40afefdf3ca666c661e213d01c542c6