Add a test utility for MediaMuxer.

This test/utility copy samples from one video and mux into another video.
It support trimming, cutting audio or video track.
It can run simply as command line like:
adb shell muxer -a -v -s 1000 -e 8000 "/sdcard/DCIM/Camera/VID_*.mp4"

bug:7991013

Change-Id: I8a2eeff3cabd001b6b2a7062d991dd076edbf22e