Add support for virtual sensors.
Rework sensorservice to allow "virtual sensors", that is
sensors that report a synthetized value based on real sensors.
the main change to sensorservice is around managing which real
sensor need to be activated and which rate to use.
The logic for all this has been moved into SensorDevice, which
essentially wraps the sensor HAL but adds two features to it:
- it keeps track of which sensors need to be activated
- it keeps track of what rate needs to be used
For this purpose an "identity" is associated with each real sensor
activation, so we can track them.
On start-up we check for gravity, linear-acceleration and
rotation-vector sensors, if they're not present in the HAL, we
synthetize them in sensor-service.
Change-Id: I841db2c1b37ef127ed571efa21732ecc5adf1800
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