allow WifiService to use tracing on user builds

Previously, we'd restricted WifiService's use of
the kernel's tracing feature to just userdebug_or_eng
builds.

This restriction was in place because the feature
had not yet been reviewed from a privacy perspective.
Now that the feature has passed privacy review, enable
the feature on all builds.

Note that other safeguards remain in place (on all
builds):
- The set of events to be monitored is configured by
  init, rather than WifiService (part of system_server).
  This privilege separation prevents a compromised
  system_server from tracing additional information.
- The trace events are kept only in RAM, until/unless
  WifiService receives a dump request. (This would happen,
  for example, in the case of adb dumpsys, or generating
  a bugreport.)

Bug: 35679234
Test: manual (see below)

Manual test details:
- flash device
- connect device to a wifi network
$ adb shell dumpsys wifi | grep rdev_connect
  [should see at least one matching line]

Change-Id: I85070054857d75177d0bcdeb9b2c95bfd7e3b6bc