hammerhead: Add SELinux policy for healthd

This adresses denials such as:

avc:  denied  { read write } for  pid=144 comm="healthd" name="rtc0"
dev="tmpfs" ino=6565 scontext=u:r:healthd:s0 tcontext=u:object_r:rtc:s0
tclass=chr_f
avc:  denied  { open } for  pid=143 comm="healthd" name="/" dev="tmpfs"
ino=5405 scontext=u:r:healthd:s0 tcontext=u:object_r:device:s0 tclass=dirile

Change-Id: Ib201cfcf3c21c9480276c0356a063701d5120810