Fix google-explict-constructor warnings in mtp.

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

Change-Id: I3a84ae7ffa03b2e23562edef95ef6abefe0ff456
diff --git a/media/mtp/MtpObjectInfo.h b/media/mtp/MtpObjectInfo.h
index 86780f1..188f21c 100644
--- a/media/mtp/MtpObjectInfo.h
+++ b/media/mtp/MtpObjectInfo.h
@@ -47,7 +47,7 @@
     char*               mKeywords;
 
 public:
-                        MtpObjectInfo(MtpObjectHandle handle);
+    explicit            MtpObjectInfo(MtpObjectHandle handle);
     virtual             ~MtpObjectInfo();
 
     bool                read(MtpDataPacket& packet);