Correctly initialize runtime sensors

For SensorManager instances associated with virtual devices with
custom sensor policy, the sensor list and mapping from handle to
sensor is only created after the first call to getSensorList()

This breaks the assumption that the sensors are static per device

Initializing the list in the constructor solves the problem

Fix: 371586534
Test: presubmit and manual with VDM demo apps
Flag: EXEMPT trivial bugfix
Change-Id: Ia4c0e19da78cd3e5e021fc4c70729a24b7347e5e