charger: Load default resources from /system

When charger resources are missing from /product, before this change,
images are loaded from /res/images/charger. After this change, charger
first checks if the resource exists at that location, and if not, use
resource under /system/etc/res/images/charger.

On devices that uses resources from /product, this change has no effect.
On devices that uses resources from /res,
- if an old image is mounted at /, charger continues to load resources
  from /res.
- if a new image is mounted at /, that is, battery_fail and
  battery_scale may not exist under /res, charger loads them from /system.
  There are no font files installed by modules defined here, so any
  device-specific font files stay at /res.

Also create virtual function wrappers over minui APIs to be mocked
later.

See next change on moving battery_scale and battery_fail images to
/system.

Test: build and inspect system/etc/res/images/charger
Bug: 143907179
Change-Id: I4895b9602274fa4a1ebb4d7d3b842622b81b7b51