Fixes https://github.com/matrix-org/dendrite/issues/3273
As we otherwise send down device list updates which are merely useful
for the user and causes tests to be flakey:
```
❌ TestPushSync/Adding_a_push_rule_wakes_up_an_incremental_/sync (10ms)
push_test.go:57: no pushrules found in sync response: {"next_batch":"s0_0_0_0_0_1_1_0_1","device_lists":{"changed":["@user-1:hs1"]}}
```
What this does: If a `PerformDeviceCreation` request is coming from
registering an account, it does **not** send device list updates, as
they are merely useful (no joined rooms, no one to inform) . In all
other cases, the behavior is unchanged and device list updates are sent
as usual.
