Unnecessary NDK requirement for apps that do not use native libs

`android_binary` builds will fail if `ANDROID_NDK_HOME` / `android_ndk_repository` aren't set even if it does not rely on native libs.

The implicit CC toolchain dependency originates [here](https://github.com/bazelbuild/rules_android/blob/main/rules/android_binary/attrs.bzl#L205), and is unconditionally used for processing in [native_libs.bzl](https://github.com/bazelbuild/rules_android/blob/main/rules/native_deps.bzl#L76-L78)

I was able to get our app to build without setting the NDK by butchering the toolchain code out, more or less, but a more elegant optionality would be great.