The main purpose is to make sure that the build doesn't fail
if the build directory contains the gtest subdirectory, although
the unittests also could be run on a device.
One doesn't need to run the ndk-build command from within the
jni subdirectory, calling it from the project top level directory
works just as well.
Also remove an unnecessary step of cd ../../../.. at the end -
since all make commands are run in a subshell, we don't need to
restore the current working directory afterwards.
