Fix /sys denials.

Labeled files linked from /sys/class/{rtc, net}.
Fixes these denials:
avc: denied { read } for pid=6413 comm="system_server" name="hctosys"
dev="sysfs" ino=10068 scontext=u:r:system_server:s0
tcontext=u:object_r:sysfs:s0 tclass=file permissive=0

avc: denied { write } for pid=4785 comm="Binder:4785_2" name="mtu"
dev="sysfs" ino=9213 scontext=u:r:netd:s0 tcontext=u:object_r:sysfs:s0
tclass=file permissive=0

Test: files labeled as expected; emulator boots without above denials.
Change-Id: I8fd8cc993ede0ddfd4404c0647a60da4867ef128