Fix: Return the correct list of available Java events

Fix: Return the correct list of available Java events

The "lttng list -j/-l" command should list the events that are currently
offered by Java application and available to be enabled.

Due to some confusion in the implementation of the corresponding agent
command response, it was actually returning the list of events that were
enabled in the tracing session.

Rectify this by sending the list of loggers of the corresponding domain
that have one or more LTTng log handlers attached. The interface method
was also renamed from listEnabledEvents() to listAvailableEvents() to
make it more representative.

Fixes: #933
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>