Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/gui/SensorManager.cpp b/libs/gui/SensorManager.cpp
index 33608b5..9b9e78f 100644
--- a/libs/gui/SensorManager.cpp
+++ b/libs/gui/SensorManager.cpp
@@ -142,7 +142,7 @@
                 mSensorManger.sensorManagerDied();
             }
         public:
-            DeathObserver(SensorManager& mgr) : mSensorManger(mgr) { }
+            explicit DeathObserver(SensorManager& mgr) : mSensorManger(mgr) { }
         };
 
         LOG_ALWAYS_FATAL_IF(mSensorServer.get() == NULL, "getService(SensorService) NULL");