No spinner while catching up on to-device msgs

### Steps to reproduce

1. Launch app for first time in a few weeks
2. UISIs, UISIs, everywhere
3. Over the next minute or so, they slowly decrypt.
4. No spinner at all to let you know that the app is working through a massive todevice encryption backlog.

### Outcome

#### What did you expect?

Some kind of spinner, if we're still frantically syncing data which is needed to decrypt history. This probably shouldn't be the main sync spinner, though, given the app is otherwise usable (at least for reading msgs). Could perhaps do spinner per UISI msg? "Getting keys " or something?

Alternatively, should SS perhaps just have a flag in its responses to let the client know when it's churned through the backlog, so we know when to display an actual UTD rather than "Getting keys"?

#### What happened instead?

No spinner, much flakiness, much sad.

### Your phone model

_No response_

### Operating system version

_No response_

### Application version

276

### Homeserver

_No response_

### Will you send logs?

Yes