tracing_service: Make AID_SYSTEM a privileged initiator

So that traces started by system_server can capture heap profiles or
stack samples for apps.

Now:

1. Apps with `android:debuggable="true"` can be profiled by shell,
   statsd and system_server.
2. Apps with `<profileable android:shell="true">` can be profiled by
   shell, statsd and system_server.
3. Apps with `<profileable android:enabled="true">` (the default value)
   can be profiled by statsd and **system_server** (this is the only
   change made by this commit), but not by shell.
4. Apps with `<profileable android:enabled="false">` cannot be profiled
   by anyone.

This basically adds AID_SYSTEM to the group of "system services"
mentioned in
https://developer.android.com/guide/topics/manifest/profileable-element

Change-Id: I584aa5d65c689d01323cad020a52a0045254e0a0