Make "track_new_devices" parameter for device trackers platform specific

Would be nice if the track_new_devices parameter listed here could be applied to separate platforms independently. Ie, keep tracking new devices in owntracks but not in bluetooth.

According to current docs, it seems as if this is a global parameter that must be included in the first defined device tracker platform. Two issues with this setup: as mentioned above, you cannot have separate setups for each platform. Also, if using the “include_dir_merge_list” directive for device tracker, there is no way to determine which platform is loaded first.