healthd: Reinitialize mChargerNames for every battery update

Booting up the device without the usb sets the usb power supply
type as UNKNOWN. Due to this mChargerNames gets incorrectly
initialized at bootup. The value of usb power supply type changes
at run-time. So it makes sense to update mChargerNames everytime
we have a battery related update.

Change-Id: I2ec9f9a420ca61814d43c316b418ce94de3691bc