Room avatar not show if room avatar is not set, in "non encrypted room"

### Steps to reproduce

1. Where are you starting? What can you see?

Create a new room, add another user, after room member increased to 2 (accepts the invitation),
Room avatar just show first letter of 2nd user, instead of his avatar.

As a comparison, desktop client nheko will show 2nd user's avatar for these kinds of room.
These kinds of room stand for one to one chat, and without encryption enabled.

If "start direct chat", then a room with encryption enabled will be created,
they are basically the same as the example I just mentioned,
the only difference is encryption (at least in my observed).

### Outcome

#### What did you expect?
What I expect is if a room have only 2 user, for example, me, and "Tom",
on my device, show this room's avatar and name as "Tom"'s avatar and name,
if Tom changed his "room display name" (/myroomnick) to "Tom2",
this room will name as "Tom2" and still have "Tom"'s avatar.

On the other hand, on Tom's device, show this room's avatar and name as "My" avatar and name.

Generally speaking, I'd wish to have something like "start direct chat" without encrypt enabled.
My these gamer not programmer friends are very confusing with encryption.
They may lose their key without backup in one day and messed up everything.
#### What happened instead?
A room have only 2 members without encryption, only show another person name's first letter, instead of avatar.

### Your phone model

Pixel 2

### Operating system version

Android 11

### Application version and app store

Element version 1.5.20 [40105200] (F-3e947e43) from F-Droid

### Homeserver

Synapse home server https://hub.docker.com/layers/matrixdotorg/synapse/latest/images/sha256-b33e1f522f37ea99a6d1b1ce99ac84b408d09fd542c16c60aa2c20720ec2ce58?context=explore

### Will you send logs?

No

### Are you willing to provide a PR?

No