sepolicy: allow hal_power_stats to read sysfs_display

avc:  denied  { read } for  name="available_disp_stats"
dev="sysfs" ino=76162 scontext=u:r:hal_power_stats_default:s0
tcontext=u:object_r:sysfs:s0 tclass=file permissive=0

Bug: 317767775
Test: dumpsys android.hardware.power.stats.IPowerStats/default
Change-Id: I272f69f4c4720eb4800a8a13ef62e1ab34cbaedf
Signed-off-by: Darren Hsu <darrenhsu@google.com>