Refine BatteryInfo and battery drawable

1. Remove "m" prefix for all public field
2. Change logic about field "discharging", now it only controlled
   by whether it is plugged in.
3. Change Paint in drawable to protected
Bug: 36862496
Test: RunSettingsLibTests
Change-Id: I2908ae6868dc877d3ce6a8a63ec17826a6dcca1d