Label ro.property_service.async_persist_write as build_config_prop

So far, it has been labeled as default_prop because there was no entry
for the sysprop in property_context. As a result, it couldn't be set by
vendor_init.

Fixing that by correctly labeling it. build_config_prop is defined as
`system_vendor_config_prop` which vendor_init can set.

Bug: 250125146
Test: adb root && adb shell ro.property_service.async_persist_write 1
adb shell getprop -Z ro.property_service.async_persist_write

shows [ro.property_service.async_persist_write]: [u:object_r:build_config_prop:s0]

Change-Id: Ib30c708c8c2693892503a8f0d590541984c2667b
diff --git a/private/property_contexts b/private/property_contexts
index 3400597..e3a0ca9 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -1143,6 +1143,7 @@
 ro.crypto.type  u:object_r:vold_status_prop:s0 exact enum block file managed none
 
 ro.property_service.version u:object_r:property_service_version_prop:s0 exact int
+ro.property_service.async_persist_write u:object_r:build_config_prop:s0 exact bool
 
 ro.vendor.redirect_socket_calls u:object_r:vendor_socket_hook_prop:s0 exact bool