Add label for time (zone) system properties

This adds a label for system properties that will affect system-wide
time / time detection logic.

The first example will be something like:
persist.time.detection_impl_version

Bug: 78217059
Test: build
Change-Id: I46044f1e28170760001da9acf2496a1e3037e48a
diff --git a/private/system_server.te b/private/system_server.te
index d3b8bcd..d7bf54a 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -795,6 +795,9 @@
 allow system_server functionfs:dir search;
 allow system_server functionfs:file rw_file_perms;
 
+# system_server contains time / time zone detection logic so reads the associated properties.
+get_prop(system_server, time_prop)
+
 ###
 ### Neverallow rules
 ###