Let system server set permissions on marker file

System server needs to create a file in /metadata/aconfig, and set its
permissions.

Bug: 328444881
Test: m
Change-Id: I30aa576e46d8963e78ff21ad328160a99bd5d523
diff --git a/private/file_contexts b/private/file_contexts
index 193c033..75ebf2c 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -872,6 +872,7 @@
 /metadata/repair-mode(/.*)?    u:object_r:repair_mode_metadata_file:s0
 /metadata/aconfig(/.*)?    u:object_r:aconfig_storage_metadata_file:s0
 /metadata/aconfig/flags(/.*)?    u:object_r:aconfig_storage_flags_metadata_file:s0
+/metadata/aconfig_test_missions(/.*)?    u:object_r:aconfig_test_mission_files:s0
 
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