record the last event received regardless of having clients or not

We only recorded the last received event (which is needed when a sensor
is activated on a connection) when there was some connection active.
This should fix an issue where sometimes the light sensor doesn't
return an event whent activated.

we also didn't need to hold the main lock while dispatching events
to clients.

Change-Id: I6c6386c040051ce205e3c0516c678e0603fa45e1