Add sysfs_batteryinfo label.

Shell user needs to be able to get current device battery_level via
/sys/class/power_supply/battery/capacity.  Create a global label and
corresponding policy for accessing this.  Rely on each device to label
the appropriate sysfs entry.

Bug: 26219114
Change-Id: I2c5ef489a9db2fdf7bbd5afd04278214b814351c