Fix: skip start trace for app that are already started.

When using trackers, call to ust_app_global_update
results in a call to ust_app_start_trace even if the ust app
trace was already enabled.

This resulted in error message from the app (-1024) complaining that the
tracing was already active on the app side.

  Error: Error starting tracing for app pid: 329376 (ret: -1024)
  ok 211 - Track command with opts: 0 -u --vpid 329376

A solution to this is to keep the enabled state inside the ua_sess and
skip the start trace if already enabled.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: Ie4198245627e9f0426948489bc9ce24d8c813b08
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>