Allow perfetto traced_probes to access tracefs on user

Allows the traced_probes daemon to access the core ftrace
functionalities on user builds. Specifically this involves:
- Whitelisting the per_cpu/ subdirectory to access:
  1) trace_pipe_raw file to allow perfetto to read the raw
     ftrace buffer (rather than the text-based /trace endpoint)
  2) cpuX/stats and cpuX/buffer_size_kb that allow to
     tune the buffer size per-cpu pipe and to get basic
     statistics about the ftrace buffer (#events, overruns)
- Whitelistiing the full event directories rather than the
  /enable files. This gives also access to the /format files
  for the events that are already enabled on user builds.
  /format files simply describe the memory layout
  of the binary logs. Example: https://ghostbin.com/paste/f8m4k

This still does NOT allow enabling the events labeled as
"_debug" (mostly events that return activity on inodes).
We'll deal with that separately as soon as we get a POC
of inode resolution and a sensible blacklist/whitelist model.

Bug: 70942310
Change-Id: Ic15cca0a9d7bc0e45aa48097a94eadef44c333f8