Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I7e061f68acdde368a3b63c029b928133646ebff2
diff --git a/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp b/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
index 6ebcdfe..aa45725 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
@@ -91,7 +91,7 @@
 
 class HWC2On1Adapter::Callbacks : public hwc_procs_t {
     public:
-        Callbacks(HWC2On1Adapter& adapter) : mAdapter(adapter) {
+        explicit Callbacks(HWC2On1Adapter& adapter) : mAdapter(adapter) {
             invalidate = &invalidateHook;
             vsync = &vsyncHook;
             hotplug = &hotplugHook;