Add userspace_reboot_log_prop

This properties are used to compute UserspaceRebootAtom and are going to
be written by system_server. Also removed now unused
userspace_reboot_prop.

Test: builds
Bug: 148767783
Change-Id: Iee44b4ca9f5d3913ac71b2ac6959c232f060f0ed
diff --git a/private/system_server.te b/private/system_server.te
index f2f1707..7b24f4b 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -1091,6 +1091,9 @@
 allow system_server password_slot_metadata_file:dir rw_dir_perms;
 allow system_server password_slot_metadata_file:file create_file_perms;
 
+# Allow init to set sysprop used to compute stats about userspace reboot.
+set_prop(system_server, userspace_reboot_log_prop)
+
 # JVMTI agent settings are only readable from the system server.
 neverallow {
   domain