Add logd.ready

logd.ready is a system property that logd sets when it is ready to
serve incoming socket requests for reading and writing logs. Clients of
logd (e.g. logcat) can use this to synchronize with logd, otherwise they
may experience a crash due to the refused socket connection to logd when
they are started before logd is ready.

Bug: 206826522
Test: run microdroid. see logcat logs are shown immediately
Change-Id: Iee13485b0f4c2beda9bc8434f514c4e32e119492
diff --git a/private/logpersist.te b/private/logpersist.te
index ab2c9c6..e151810 100644
--- a/private/logpersist.te
+++ b/private/logpersist.te
@@ -15,6 +15,7 @@
 
   control_logd(logpersist)
   unix_socket_connect(logpersist, logdr, logd)
+  get_prop(logpersist, logd_prop)
   read_runtime_log_tags(logpersist)
 
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