Only show "charging" if the battery is really charging.

If there's a charge problem (battery health/temperature out
of spec, or low charging current), the lockscreen and status
bar should not indicate that charging is occurring.

(Previously we showed "Charging" and a lightning bolt any
time the device was plugged in.)

Bug: 7509976
Change-Id: I27ec0e2cec1307fbc6ef262dd5181a4db6d3bed0