Sysprop for the count of active OOME tracing sessions

In order for ART code to call perfetto DataSource::Trace() we need to
wait for all data source instances to have completed their setup. To do
so, we need to know how many of them exist.

This introduces a new sysprop traced.oome_heap_session.count, writeable
by perfetto traced and readable by apps and system_server that can be
used to communicate this.

See go/art-oom-heap-dump for more details

Test: manual, atest HeapprofdJavaCtsTest
Bug: 269246893
Change-Id: Ib8220879a40854f98bc2f550ff2e7ebf3e077756
diff --git a/private/traced.te b/private/traced.te
index 3029094..171e092 100644
--- a/private/traced.te
+++ b/private/traced.te
@@ -60,6 +60,11 @@
 set_prop(traced, system_trace_prop)
 # Allow to lazily start producers.
 set_prop(traced, traced_lazy_prop)
+# Allow tracking the count of sessions intercepting Java OutOfMemoryError
+# If there are such tracing sessions and an OutOfMemoryError is thrown by ART,
+# the hprof plugin intercepts the error, lazily registers a data source to
+# traced and collects a heap dump.
+set_prop(traced, traced_oome_heap_session_count_prop)
 
 # Allow traced to talk to statsd for logging metrics.
 unix_socket_send(traced, statsdw, statsd)