Move Android NDK installation to a composite action

[kendalharland]

The new steps to install the Android NDK in swift-toolchain.yml would be great candidates for a composite action that takes the NDK version and hash as inputs, and optionally sets the ANDROID_NDK and ANDROID_NDK_ROOT environment variables. Android-Determine.cmake will derive CMAKE_ANDROID_NDK from these values if set (See these lines), allowing us to remove the explicit matrix.extra_flags entries we use to pass this value to cmake.

Here's an example workflow run where I tested this behavior while building libxml2: https://github.com/thebrowsercompany/swift-build/actions/runs/9704486640/job/26784869710

CC @hyp @compnerd

[compnerd]

Why not use nttld/setup-ndk?