Don't send device list updates upon registration (#3307)

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.

.github/codecov.yaml

@@ -7,7 +7,7 @@ coverage:
     project:
       default:
         target: auto
-        threshold: 0%
+        threshold: 0.1%
         base: auto
         flags:
           - unittests