Always cache the default device sensors.

Since ag/24955733 the native SensorManager may be initialized with
a VirtualDevice's sensors instead of the default device ones and this
is WAI. However, this breaks an assumption in SystemSensorManager.java

SystemSensorManager's mFullSensorsList must always hold the default
device's sensors. Any virtual device's sensors are stored in
mFullRuntimeSensorListByDevice and the correct sensors are returned
from getSensorsList based on the manager's context.

If mFullSensorsList is initialized with virtual sensors, then there
is no way to get the default device's sensors anymore.

Fix: 315295201
Test: manual with the VDM test app
Change-Id: I8add9f4ece2fdd6d83f6388990b1772987eae939