Fix a few issues in MediaMuxerTest.

- The test refers to hard coded paths in /sdcard that are
  not guaranteed to exist, besides it's bad practice since
  a failing test can leave residual files behind.

- Fix broken lat/long parsing that would only work in the
  western hemisphere.

NOTE: The delta check for latitudes and longitudes look
broken as well, but I didn't dig into it.

Change-Id: I6b164ede18e1847f17353166fcdd9d8e091c397b