Add boot_status_prop for boot completed props

Assigning a new context boot_status_prop for following two properties:
- sys.boot_completed
- dev.bootcomplete

Bug: 154885206
Test: boot cf_x86 and crosshatch, see no denials
Change-Id: Ieadabf90a9a1b54b52a1283bd648c11c95d558dd
Merged-In: Ieadabf90a9a1b54b52a1283bd648c11c95d558dd
(cherry picked from commit 2973c9605555c119243ef85119576b2d01a12060)
diff --git a/private/system_server.te b/private/system_server.te
index 6c1fa9a..4fc507f 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -620,6 +620,7 @@
 set_prop(system_server, exported_pm_prop)
 set_prop(system_server, socket_hook_prop)
 set_prop(system_server, audio_prop)
+set_prop(system_server, boot_status_prop)
 userdebug_or_eng(`set_prop(system_server, wifi_log_prop)')
 
 # ctl interface
@@ -1163,3 +1164,5 @@
 
 # Do not allow any domain other than init or system server to set the property
 neverallow { domain -init -system_server } socket_hook_prop:property_service set;
+
+neverallow { domain -init -system_server } boot_status_prop:property_service set;