Allow system server to lock system files.

ART generically locks profile files, and this avoids
special casing the ART code for read-only partitions.

An example on how ART does it:
https://android-review.googlesource.com/c/platform/art/+/958222/3/runtime/jit/jit.cc#731

Bug: 119800099
Test: system server locking a system file, no denial
Change-Id: I4339f19af999d43e07995ddb77478a2384bbe209
diff --git a/private/system_server.te b/private/system_server.te
index 29ef906..4bf8f8e 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -648,6 +648,9 @@
 allow system_server system_file:dir r_dir_perms;
 allow system_server system_file:lnk_file r_file_perms;
 
+# ART locks profile files.
+allow system_server system_file:file lock;
+
 # LocationManager(e.g, GPS) needs to read and write
 # to uart driver and ctrl proc entry
 allow system_server gps_control:file rw_file_perms;