Take default for background activity starts from DeviceConfig.

We've modified the implementation to use the value set by DeviceConfig
if the user has not touched the developer options setting (change
fb227077b8652b817a094d37a5215ad7466ee603). We want the Settings screen
to reflect this default until the user explicitly sets it.

Additionally if the user disables developer settings we want to go
back to using whatever DeviceConfig sets.

Bug: 129533810
Test: croot && make RunSettingsRoboTests \
ROBOTEST_FILTER=AllowBackgroundActivityStartsPreferenceControllerTest

Change-Id: I73901a22da928f9a6924b30882b26494cdcd94bc