[plugins/battery] Determine empty battery based on percentage instead of remaning time

timeToEmpty in UPower is unreliable, e.g. it's always 0 on the PinePhone.

Instead warn when the percentage drops below 15%. That's how it is for Android anyway.
1 job for master in 5 minutes and 37 seconds (queued for 70 minutes and 55 seconds)
Status Job ID Name Coverage
passed #93776
linux kf5-qt5 qt5.15