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/MtpPacket.h b/media/mtp/MtpPacket.h
index 037722a..cca2783 100644
--- a/media/mtp/MtpPacket.h
+++ b/media/mtp/MtpPacket.h
@@ -35,7 +35,7 @@
     size_t              mPacketSize;
 
 public:
-                        MtpPacket(int bufferSize);
+    explicit            MtpPacket(int bufferSize);
     virtual             ~MtpPacket();
 
     // sets packet size to the default container size and sets buffer to zero