Companion PR to https://github.com/koreader/koreader-base/pull/884
* Basically flags devices known to be stable when using PxP inversion.
* Plus, random fix for #4870 ;).
* A few FrontLight tweaks & cleanups on Kobo:
* Moved the Kobo-specific startup status insanity to Kobo-specific init
* Made turnOff/turnOn frontlight do a smooth ramp down/up
* On Kobo, use turnOff/turnOn for suspend/resume, to get that smooth toggle
* On Kobo, for NaturalLight w/ a mixer, only set warmth for setWarmth, and only set Brightness for setBrightness, otherwise, it tried to set both with not in-sync values, which made the FL widget jittery.
* Start battery stat plugin
* BatteryStat & kobolight
* Several minor improvements
* Remove a useless function
* flush settings
* Some review feedbacks
* Resolve review comments
* Remaining Minutes -> Remaining Hours
* Add dump_file
* typo
* realpath
* realpath on folder
* Remove useless os.time()
* Resolve review comments
* warning
* Add BatteryStat.debugging flag
* treat log as txt
* Minor improvement
* Charging hour should be positive
* Use warn instead of info
* onSuspend in Kobo
* Charging events for kobo and kindle
* More events
* dumpOrLog
* Warnings
* Typo
* More space
* Singleton
* slightly format change
* BatteryStat singleton
* Init
* Remove debugging flag
* sleeping percentage is still negative
* Read settings
* Do not need to change was_suspending and was_charging
* Typo
* Remove debugging flag
* Not charging should happen before suspend
* Resolve review comments
* was_suspend and was_charging should be updated each time in onCallback()