Rename Java Agent event names to "event"

Previously, separate event names were used for events targeting
a root session daemon or a user one. Those were "sys_event" and
"user_event", respectively.

Now that the Java side only sends one event type, we can rename
it to simply "event".

This patch is dependant on a similar update on the lttng-tools
side.

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