Improve NDK version detection.

The script assert_ndk_version.sh used to require that the user
either define ANDROID_NDK_ROOT or ndk-build be in the path
so that ANDROID_NDK_ROOT can be inferred.

But since we are always calling this script from within an
Android.mk file, ndk-build already defines a variable called
NDK_ROOT, which we can pass to the script, thereby simplifying it
and making it more robust.

Change-Id: I31d66529a36a93e760e0dd0f17cfe61428fbac55