ReadTimer: Small QoL fixes
ReadTimer: Small QoL fixes

* Never show negative relative time values.
* Make sure the alarm fires on resume if it expired during suspend.

Fix #8031

poire-z/koreader,Frenzie/koreader,koreader/koreader,NiLuJe/koreader,NiLuJe/koreader,poire-z/koreader,Frenzie/koreader,koreader/koreader