Please provide a way to pass android.ndkPath

When building for android, we've the case that SDK and NDK are not installed using Android Studio (automatted CI usecase). So there's no NDK side by side folder inside the SDK and the NDK lives in a directory of it's own.

To make this work, it is required to set android.ndkPath from within [https://developer.android.com/studio/projects/configure-agp-ndk|build.gradle]. This one is currently not exported by androidbuildqt and needs extra tweaking in cmake instead.

If the ndkPath is not set, the build succeeds, but stripping the debug symbols fails with a warning, as the llvm-strip binary is not found.