Allow traced to notify traceur via property

This CL introduces allows traced to set the
sys.traceur.trace_end_signal property at the end
of the tracer. In turn that property notifies the
the Traceur app.
This is to allowing Traceur to be killed during
a long-trace and avoid wasting resources making
it a persistent service.
See aosp/886616 for the matching traceur change.

Test: manual
Bug: 116754732
Change-Id: I89e2f02b3f973813ce8ff3507d397a06502f84c1
diff --git a/private/property_contexts b/private/property_contexts
index 4eb10dd..8024954 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -79,6 +79,7 @@
 pm.                     u:object_r:pm_prop:s0
 test.sys.boot.reason    u:object_r:test_boot_reason_prop:s0
 sys.lmk.                u:object_r:system_lmk_prop:s0
+sys.trace.              u:object_r:system_trace_prop:s0
 
 # Boolean property set by system server upon boot indicating
 # if device owner is provisioned.