QtQ4A: Drop android_ndk_version from config file

Why?

With QtGP 1.2 we can just add an extraCMakeArguments property that uses the value in ANDROID_NDK_ROOT instead of digging around in the SDK directory. Less stuff to configure and keep up to date is nice.

What?

Remove android_ndk_version from the config file. Don't link the NDK under the SDK, instead add extraCMakeArguments to QtBuild that sets ANDROID_NDK_ROOT for the build (similar to how we do it in QtGP RTA).