set NDK_PLATFORM_LEVEL & NDK_TOOLCHAIN_VERSION only in jni/Application.jni

This makes the external/Makefile get the values of these two variables from
the official Android place to set them, keeping the build systems in sync.

NDK platform level, aka APP_PLATFORM, is equivalent to minSdkVersion.

http://stackoverflow.com/questions/14551113/android-ndk-multiple-api-levels