Fixed one show stopper relating tablet's rotation and messenger.

How to reproduce
================

1. Select a directory.
2. Select a file to play.
3. Pause.
4. Rotate a tablet.
5. Slide the record (or the head).

What I got?
===========

NullPointerException for MainActivity.outgoingMessenger.

Why?
====

When a user rotates a tablet, outgoingMessenger will get null.
MainActivity.onResume() also does not care about it.

Why did not I find this bug?
============================

Even after a user paused music, MainActivity.outgoingMessenger stayed
non-null. And that messenger might be alive. I did not test sliding with
rotating.
Enhanced to show timestamp in sliding the record or the head.