sensorservice: customize sensor fusion mag filter via prop

When large magnetic fields values get filtered out by the Sensor
Fusion algorithm the fused sensors (rotation vector, gravity, linear
acceleration etc) do not produce any output. As a result the CTS tests
fail if the compass is uncalibrated on some devices.

This change allows the magnetic field filter to be tweaked on a
per-device basis to allow the fused sensors to operate even when the
compass in uncalibrated.

Change-Id: I608b307e8b7712e806315a6181c19130ec17c847