inactivity: Use LVGL inactivity timers

[Riksu9000]

Replace custom FreeRTOS inactivity timers with LVGL inactivity timers.

There may be some more optimizations possible, for example the restore brightness message can probably be replaced with lv_disp_trig_activity(nullptr);

- [ ] Fix an issue where setting the timer to the exact timeout setting causes the watch to go to sleep at the time the timer expires. Looks like this isn't a new issue and shouldn't be blocking.

[github-actions[bot]]

Build size and comparison to main:	Section	Size	Difference
text	406488B	-148B
data	940B	0B
bss	53568B	0B