FailureDetector: use robust timeout checks for motor failure detection

[MaEtUgR]

Solved Problem

When checking the code to write docs for #25756 I found:

• the timestamp gets saved also when not armed
• the timeout checks for ESC telemetry timeout and ESC undercurrent hysteresis use subtraction on unsigned timestamps which lead to hard to find wrapping issues before.

Solution

• Save timestamp only for the scope needed
• change timeout checks now - hopefully_older_time > timeout → now > hopefully_older_time + timeout

Changelog Entry

FailureDetector: use robust timeout checks for motor failure detection

Test coverage

No specific testing at the moment. Plaese review for mistakes. This is just a small improvement. I'll more thoroughly test motor failure detection and do more changes but found it was worth to fix now that I see it.