sepolicy: allow hal_usb_default to read sysfs_usb_supply

Grant file read access to hal_usb_default context to read
from the syfs_usb_supply file context. This allows the USB
HAL to be able to read from /sys/class/power_supply/usb/*.

Change-Id: I8b14bcd15f809f1536f948345770d46a637db449