MTP: Clean up MtpServer initialization and threading:

Move thread from native to Java code
Remove the stop() method (the thread will exit on its own)

Change-Id: Ib897c3630162f82669a4ee8c187e8172da50b29d
diff --git a/media/mtp/MtpServer.cpp b/media/mtp/MtpServer.cpp
index 9085f10..4047e2e 100644
--- a/media/mtp/MtpServer.cpp
+++ b/media/mtp/MtpServer.cpp
@@ -237,6 +237,8 @@
 
     if (mSessionOpen)
         mDatabase->sessionEnded();
+    close(fd);
+    mFD = -1;
 }
 
 void MtpServer::sendObjectAdded(MtpObjectHandle handle) {