More information in sensor bugreports.

i) Log last 20 activations/deactivations of sensors.
ii) Increase logging upto 50 events for accelerometer, step_counter and significant motion.

Change-Id: I3966ed0abda8b705f3d16e94a9370fa1b97baa57
diff --git a/services/sensorservice/SensorDevice.cpp b/services/sensorservice/SensorDevice.cpp
index 6267a4c..dd1bccf 100644
--- a/services/sensorservice/SensorDevice.cpp
+++ b/services/sensorservice/SensorDevice.cpp
@@ -89,7 +89,7 @@
         result.appendFormat("handle=0x%08x, active-count=%zu, batch_period(ms)={ ", list[i].handle,
                             info.batchParams.size());
         for (size_t j = 0; j < info.batchParams.size(); j++) {
-            BatchParams params = info.batchParams.valueAt(j);
+            const BatchParams& params = info.batchParams.valueAt(j);
             result.appendFormat("%4.1f%s", params.batchDelay / 1e6f,
                                 j < info.batchParams.size() - 1 ? ", " : "");
         }