healthd: fix /sys denials.

Label /sys/class/power_supply/* appropriately and give healthd read
permissions to that directory.

Fixes this denial:
avc: denied { read } for pid=1386 comm="healthd" name="power_supply"
dev="sysfs" ino=2562 scontext=u:r:healthd:s0
tcontext=u:object_r:sysfs:s0 tclass=dir permissive=0

Bug: 72437093
Test: emulator boots with no denials from healthd
Change-Id: Ie3853cb5e9167fcd70f393ff589971ad6212c580