Fix google-explicit-constructor warnings in surfaceflinger.

* Add explicit keyword to conversion constructors,
  or use NOLINT for implicit converters.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I94f511c3be31dc22022db6466ee9ed0c6be2bc8e
diff --git a/services/surfaceflinger/DisplayHardware/HWC2.h b/services/surfaceflinger/DisplayHardware/HWC2.h
index 8ab61e9..1aa4205 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2.h
+++ b/services/surfaceflinger/DisplayHardware/HWC2.h
@@ -56,7 +56,7 @@
 class Device
 {
 public:
-    Device(hwc2_device_t* device);
+    explicit Device(hwc2_device_t* device);
     ~Device();
 
     friend class HWC2::Display;