debian: Run tracing service under a dedicated user

Run the 'traced' daemon under a dedicated user under Debian so that
access to the consumer socket can be gated on being a member of the
'perfetto-traced' group.

Change-Id: Ib8a2f1a3a0e3b3aef50f400bde40fa5b182ef6a7
diff --git a/debian/postrm b/debian/postrm
new file mode 100755
index 0000000..20c9767
--- /dev/null
+++ b/debian/postrm
@@ -0,0 +1,3 @@
+#!/bin/sh
+set -e
+rm -f /tmp/perfetto-consumer /tmp/perfetto-producer