Add /sys/kernel/memory_state_time to sysfs_power.

This allows system_server to access it for determining battery stats
(see KernelMemoryBandwidthStats.java).

batterystats-wo: type=1400 audit(0.0:429): avc: denied { read } for name="show_stat" dev="sysfs" ino=48071 scontext=u:r:system_server:s0 tcontext=u:object_r:sysfs:s0 tclass=file permissive=0

Bug: 72643420
Bug: 73947096

Test: Denial is no longer present.
Change-Id: Ibe46aee48eb3f78fa5a9d1f36602c082c33036f7

(cherry picked from commit a8b3634d)