perfetto: Add setting to enable/disable traced

This will help us run P/H experiments by controlling the
whether traced runs through P/H.

This will allow to gradually roll out traced and, in an
emergency, remotely disable it.

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.
Bug: b/71737179
Bug: b/74383547
Change-Id: I1f564421d9abae14d7d80769e9517eb363dae33a
Merged-In: I1f564421d9abae14d7d80769e9517eb363dae33a