Mozilla bug 698328 - AudioTrack.cpp backend for Android.

This uses android/log.h to check that we are on Android in configure.ac, and is
rebased on top of the backend switching changes.
Merge pull request #15 from padenot/audiotrack

audiotrack: New backend for Android (from https://bugzilla.mozilla.org/show_bug.cgi?id=839415)
audiotrack: cubeb_ops must be first member of struct cubeb.
opensl,directsound: initialize context ops.
Add audiotrack backend in cubeb.c