Fixes to SystemCapabilityManager
Fixes to SystemCapabilityManager

* When sending a GetSystemCapability when subscribing or unsubscribing, we don't need to manually invoke the observer. This is automatically done in the `sdl_sendGetSystemCapabilityWithType:` method by saving the capability that was retrieved.

* Before calling observers, run a loop to check if any observers have gone to `nil` and remove them, then unsubscribe if we have no more observers for that SC type.

smartdevicelink/sdl_ios,smartdevicelink/sdl_ios,smartdevicelink/sdl_ios,smartdevicelink/sdl_ios,smartdevicelink/sdl_ios