BatteryService: Allow power supplies to switch type between AC and USB

Accomodate power_supply drivers that switch between MAINS and USB type
according to the current power source.  Re-read the type attribute when the
power supply is online.

Switch to String8 type for strings stored locally.

Change-Id: Iacce49bf3ad85f35a7295a54df43aff7f94f3100