s4-common: [WIP] use offmode_charging from m7

Not ready - two issues:
1) last_kmsg is not generated when offmode charging
2) Images charging_00.png and charging_05.png do not display

* Borrow offmode_charging and choice_fn from m7 to enable charging
  images support
* Remove power_test and detect_key as they don't seem to be needed
* Shrink charging images from m7 from 1080p to 540p (fireball, ville)
  and 720p (evita, jewel).

Change-Id: Ida40ad57ceb0fb5669f2adb840f31c7b0e242cca