fastcharge: New enhancements

* Add 1800 mA level
* Add "fake_charge_ac" hack. If fast charge is on with fake_charge_ac = 1,
  device will show current source as AC instead of USB.
* By default, fake_charge_ac is disabled.

Sincere thanks to mankindtw@xda for idea.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>