Prevent paused ThresholdSensorImpl event delivery.

With this change, a paused ThresholdSensorImpl will no longer deliver
events. This can otherwise happen if a ThresholdSensorImpl attempts
to unregister itself, but an event comes before the de-registration
actually occurs.

This can occur because ThresholdSensorImpl relies on an
AsyncSensorManager.

Fixes: 155135632
Test: atest SystemUITests
Change-Id: I26f743016cba2383e853436ac41463a7677692a9