GNOME is switching to Tracker 3.x by default (see Initiatives#17 (closed)) however Photos is not ready to switch this cycle.
We want to avoid having two Tracker daemons indexing the same content as much as possible. This commit means that Photos will start private instances of Tracker 2.x daemons, and the daemons will be tied to the lifetime of the Photos app. Distros can disable Tracker 2.x by default and it will only activate when needed by Photos.
This also allows the Photos Flatpak to run Tracker 2.x daemons inside the sandbox, ensuring it works on systems which don't have Tracker 2.x available.