Fix google-explicit-constructor warnings in  HardwareAPI.

* Add explicit keyword to conversion constructors.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I48521040417aec068795d45aaa2a3870596f9a96
diff --git a/include/media/hardware/HardwareAPI.h b/include/media/hardware/HardwareAPI.h
index 2c50ad6..cecf715 100644
--- a/include/media/hardware/HardwareAPI.h
+++ b/include/media/hardware/HardwareAPI.h
@@ -270,7 +270,7 @@
     // output: fill out the MediaImage fields
     MediaImage sMediaImage;
 
-    DescribeColorFormatParams(const DescribeColorFormat2Params&); // for internal use only
+    explicit DescribeColorFormatParams(const DescribeColorFormat2Params&); // for internal use only
 };
 
 // A pointer to this struct is passed to OMX_GetParameter when the extension