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/MtpStorageInfo.h b/media/mtp/MtpStorageInfo.h
index 35a8189..3b70675 100644
--- a/media/mtp/MtpStorageInfo.h
+++ b/media/mtp/MtpStorageInfo.h
@@ -36,7 +36,7 @@
     char*               mVolumeIdentifier;
 
 public:
-                        MtpStorageInfo(MtpStorageID id);
+    explicit            MtpStorageInfo(MtpStorageID id);
     virtual             ~MtpStorageInfo();
 
     bool                read(MtpDataPacket& packet);