healthd: Detect power supply type for all charger devices

Power supply type is not determined via the device name,
hence iterate over all available devices in the subsystem
node and read the type from device file.

  shell@hammerhead:/ $ ls /sys/class/power_supply
  ac
  batt_therm
  battery
  touch
  usb
  wireless

<4>[ 3184.867782] healthd: touch: Unknown power supply type
<4>[ 3184.868039] healthd: batt_therm: Unknown power supply type
<6>[ 3184.880506] healthd: battery l=89 v=4181 t=25.5 h=2 st=2 c=-288 chg=u
<4>[ 3184.890362] healthd: touch: Unknown power supply type
<4>[ 3184.890549] healthd: batt_therm: Unknown power supply type
<6>[ 3184.899419] healthd: battery l=89 v=4181 t=25.5 h=2 st=2 c=-59 chg=u
<4>[ 3184.908756] healthd: touch: Unknown power supply type
<4>[ 3184.908984] healthd: batt_therm: Unknown power supply type
<6>[ 3184.919672] healthd: battery l=89 v=4181 t=25.6 h=2 st=2 c=-25 chg=u

Change-Id: I863bfab95193899460237b51997e0418eeb4ee2c