Fix google-explicit-constructor warnings in libhardware

Bug: 28341362
Change-Id: I29a56734e54740e4b15fe2f6cacbbd867163ae79
Test: build with clang-tidy
diff --git a/modules/usbcamera/HotplugThread.h b/modules/usbcamera/HotplugThread.h
index a13adb7..625dcb4 100644
--- a/modules/usbcamera/HotplugThread.h
+++ b/modules/usbcamera/HotplugThread.h
@@ -36,7 +36,7 @@
 class HotplugThread : public android::Thread {
 
     public:
-        HotplugThread(CameraHAL *hal);
+        explicit HotplugThread(CameraHAL *hal);
         ~HotplugThread();
 
         // Override below two methods for proper cleanup.