Allow adbd to set/get persist.adb props, system_server to get.

Bug: b/111434128

Test: getprop persist.adb.wifi.guid
Change-Id: If211c2d00724f62a201dd9c19afc9e894001069f
diff --git a/private/system_server.te b/private/system_server.te
index ec79319..2f0b2f0 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -831,6 +831,9 @@
 allow system_server adbd:fd use;
 allow system_server adbd:unix_stream_socket { getattr getopt ioctl read write shutdown };
 
+# Read persist.adb.wifi. properties
+get_prop(system_server, shell_prop)
+
 # Allow invoking tools like "timeout"
 allow system_server toolbox_exec:file rx_file_perms;