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.

clientapi/routing/register.go

@@ -630,6 +630,7 @@ func handleGuestRegistration(
 		AccessToken:       token,
 		IPAddr:            req.RemoteAddr,
 		UserAgent:         req.UserAgent(),
+		FromRegistration:  true,
 	}, &devRes)
 	if err != nil {
 		return util.JSONResponse{
@@ -919,6 +920,7 @@ func completeRegistration(
 		DeviceID:          deviceID,
 		IPAddr:            ipAddr,
 		UserAgent:         userAgent,
+		FromRegistration:  true,
 	}, &devRes)
 	if err != nil {
 		return util.JSONResponse{