Adding libopus source and Android.mk

Adding libopus source code and Android.mk. The source is based
on latest stable release libopus 1.1.

Change-Id: I0841894ee0766829e6d0e1b8379e6f6f87b0852f
diff --git a/README.android b/README.android
new file mode 100644
index 0000000..e09b6ce
--- /dev/null
+++ b/README.android
@@ -0,0 +1,2 @@
+* current source is based on libopus 1.1 stable (http://downloads.xiph.org/releases/opus/opus-1.1.tar.gz)
+* libopus is BSD-licensed - http://www.opus-codec.org/license/