leaks Event classes on every sensor change callback

[Martin Pitt]

./android/default/default_ubuntu_application_sensor.cpp creates a new *Event object for every sensor change, like e. g.

                ubuntu::application::sensors::AccelerometerEvent::Ptr ev(
                    new ubuntu::application::sensors::AccelerometerEvent(
                        reading->timestamp,
                        reading->acceleration[0],
                        reading->acceleration[1],
                        reading->acceleration[2])
                        );

                on_accelerometer_event(
                    make_holder(ev), this->context
                    );

but this never gets deleted, so this is a rather big memory leak (as at least on my device accelerometer events happen several times a second even when not moving it).

Ricardo and I quickly discussed how the API should behave -- whether Event* can potentially live forever and need an explicit _free() method (that would be new API), or whether the getters for Event* are only valid during the callback (on_*_event) and immediately freed. We both agreed that the latter is sufficient and much simpler (clients don't need to remember to free events every time).

So this needs some "delete ev;", and presumably this make_holder() stuff can go away entirely.