Allow preventing unrestricted Apps from being Stopped

This CL adds a flag to allow toggling the behavior on whether or not
the foreground service task manager should show a stop button for
unrestricted apps.

Bug: 230449539
Test: Manual - device_config put systemui \
      show_stop_button_for_user_allowlisted_apps true|false
Test: atest FgsManagerControllerTest
Change-Id: I4cfe54933d7c6807a7ffbac9ee709c7d8d6b83d3
Merged-In: I4cfe54933d7c6807a7ffbac9ee709c7d8d6b83d3