libiio: Initial Android.bp build file

This introduces the required build logic to create libiio.so
in the Android build system.

It adds:
 - a blueprint file;
 - an explicitly generated iio-config.h, as this is ordinarily produced
   via CMake, and this doesn't seem possible to replicate trivially under
   Soong;
 - an include file for exporting.

BUG=b:128636517
Test: mmma external/libiio
Signed-off-by: Enrico Granata <egranata@chromium.org>

Change-Id: Id392839a79ea8c45ed5d45381cdc56ff4be68f3d
3 files changed