temp/upower: sync icon and warn for low battery level

!2193 (merged) updated the default battery levels for warnings and actions in the pinephone and the pinetab. Since then I have gotten a much better behavior, as it avoided my pinephone from dying because of low batter levels without me realizing. However, since then the battery icon in phosh was never updated to show a "caution" battery level. Even after the level being below "low" and a notification having been sent, the icon remained as before.

I tackled this down to upower and have just sent a MR upstream to fix the issue. However, as there already exists a temp/upower in this repo, I thought it would make sense to have this also integrated here. At least for me, I pay much more attention to visual feedback than to notifications, so I missed the need to charge the phone a couple of times and it turned itself off before I could realize.

I have tested this in my pinephone with phosh and now below 20% the battery icon updates to the corresponding symbol.