build: Build net_test_osi using GN + ninja

This CL adds support to build the net_test_osi target using GN + ninja.
The BUILD.gn file now pulls in gtest as a dependency. Missing libc
includes have been added to sources that prevented compilation on
Goobuntu (14.04).

The osi/include/log.h header has been modified to conditionally call the
Android log utilities vs plain-old printf on non-Android builds.

BUG=21339022

Change-Id: If3c356360c56c63b3bf551b41dc8d3269d8e7e09