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

private/genfs_contexts

@@ -114,6 +114,7 @@ genfscon sysfs /power/state u:object_r:sysfs_power:s0
 genfscon sysfs /power/wakeup_count u:object_r:sysfs_power:s0
 genfscon sysfs /power/wake_lock u:object_r:sysfs_wake_lock:s0
 genfscon sysfs /power/wake_unlock u:object_r:sysfs_wake_lock:s0
+genfscon sysfs /kernel/memory_state_time u:object_r:sysfs_power:s0
 genfscon sysfs /kernel/ipv4 u:object_r:sysfs_ipv4:s0
 genfscon sysfs /kernel/notes u:object_r:sysfs_kernel_notes:s0
 genfscon sysfs /kernel/uevent_helper u:object_r:sysfs_usermodehelper:s0