polling_device_watcher: fix race between construction and start
polling_device_watcher: fix race between construction and start

Device watchers do not notify for devices that already exist when they
are constructed (e.g. win_event_device_watcher), but
polling_device_watcher was also not notifying if devices were added
between construction and start. This aligns polling_device_watcher
with win_event_device_watcher

IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense