UI: janky border behavior when engaged under minSteerSpeed

### Describe the bug

We allow cars with a non-zero `minSteerSpeed` to engage below that speed, with a `belowSteerSpeed` event. This puts up a relatively unobtrusive notice in the UI.

However, the UI border color is still green, and it still goes gray when `steeringPressed`, as if openpilot were trying to steer but the driver is overriding. But there's no actual openpilot steering to override.

Since the driver is having to actually steer the whole time, this creates visually janky and meaningless UI border flickering between green and gray.

Perhaps we should treat `belowSteerSpeed` as `SelfdriveState.OpenpilotState.overriding` the whole time?

### Provide a route where the issue occurs

f39cf149898833ff/0000008f--e01634920e

### openpilot version

3c28188d7a6b433211d665f5482720275088d7d3

### Additional info

https://github.com/user-attachments/assets/96c4faea-61aa-4ee2-a2db-c3d081e33138