Uses 4-th order low-pass for extracting gravity.

Most accelerometers have 8-bits accuracy so we beed to
reject 48dB in thestop-band, which requires a 4-th order
filter at the cut-off frequency we're using.

Change-Id: Ic00421d38d751641f86b1f3ad7663e6b44a91198
diff --git a/services/sensorservice/GravitySensor.h b/services/sensorservice/GravitySensor.h
index f9850b7..ff3bea7 100644
--- a/services/sensorservice/GravitySensor.h
+++ b/services/sensorservice/GravitySensor.h
@@ -37,7 +37,7 @@
     double mAccTime;
 
     SecondOrderLowPassFilter mLowPass;
-    BiquadFilter mX, mY, mZ;
+    CascadedBiquadFilter mX, mY, mZ;
 
 public:
     GravitySensor(sensor_t const* list, size_t count);