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.

userapi/api/api.go

@@ -379,6 +379,10 @@ type PerformDeviceCreationRequest struct {
 	// update for this account. Generally the only reason to do this is if the account
 	// is an appservice account.
 	NoDeviceListUpdate bool
+
+	// FromRegistration determines if this request comes from registering a new account
+	// and is in most cases false.
+	FromRegistration bool
 }
 
 // PerformDeviceCreationResponse is the response for PerformDeviceCreation
@@ -803,6 +807,10 @@ type PerformUploadKeysRequest struct {
 	// itself doesn't change but it's easier to pretend upload new keys and reuse the same code paths.
 	// Without this flag, requests to modify device display names would delete device keys.
 	OnlyDisplayNameUpdates bool
+
+	// FromRegistration is set if this key upload comes right after creating an account
+	// and determines if we need to inform downstream components.
+	FromRegistration bool
 }
 
 // PerformUploadKeysResponse is the response to PerformUploadKeys