Building on top of @svanderburg's android work, I have since a while an expression for android's ndk. Until I find the time to polish it to a mergeable state, I thought at least I would let people know of its existence in my branch (cpages/nixpkgs@master...android).
I use it together with an env in this form:
droidEnv = myEnvFun {
name = "droid";
buildInputs = [ jdk ant androidsdk_4_1 androidndk which gnumake ];
extraCmds = ''
export ANDROID_HOME=${androidenv.androidsdk_4_1}/libexec/android-sdk-linux
export PATH=\$PATH:${androidndk}
export GNUMAKE=${gnumake}/bin/make
export NDK_HOST_AWK=${gawk}/bin/gawk
'';
};