Allow system server to set persist.traced.enable

To enable/disable the traced and traced_probes deamons remotely we would
like system server to be able to set persist.traced.enable.
See also ag/3736001.

Denial:
selinux: avc: denied { set } for
property=persist.traced.enable
pid=1606 uid=1000 gid=1000
scontext=u:r:system_server:s0
tcontext=u:object_r:default_prop:s0 tclass=property_service
permissive=0\x0a

Run:
$ adb shell 'ps -A | grep traced'
Should see traced.
$ adb shell 'settings put global sys_traced 0'
$ adb shell 'ps -A | grep traced'
Should no longer see traced.

Test: See above.
Change-Id: I245b7df3853cabeb0e75db41fb4facaa178ab8f1