Move and rename `get_devices_with_keys_by_user` (#8204)

* Move `get_devices_with_keys_by_user` to `EndToEndKeyWorkerStore`

this seems a better fit for it.

This commit simply moves the existing code: no other changes at all.

* Rename `get_devices_with_keys_by_user`

to better reflect what it does.

* get_device_stream_token abstract method

To avoid referencing fields which are declared in the derived classes, make
`get_device_stream_token` abstract, and define that in the classes which define
`_device_list_id_gen`.