ust-app: pass down trigger object instead of event-rule

ust-app: pass down trigger object instead of event-rule

This is necessary since the conditions can contain capture descriptor
related to the event rule. The trigger object allows for access to both
conditions and actions.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I65452340145278b52d897c9d53c402b22b8941d8
Depends-on: lttng-ust: I5a800fc92e588c2a6a0e26282b0ad5f31c044479