Also provide Android.mk to support ndk-build

# Problem

Oboe is typically used as a submodule or built along with app or library. Oboe supports CMake only. However, ndk-build is still one of the main ways to build C++ code for Android projects. It is difficult to incorporate oboe into these projects and libraries that use ndk-build. This is because mixing CMake and ndk-build is [not yet supported](https://github.com/android/ndk/issues/863).

# Workaround

Users of oboe can translate CMakeLists.txt to Android.mk and keep it under their app / library. We see that people are already [doing that](https://github.com/google/oboe/issues/579) in their projects. It is easy to forget to update this mk file as and when newer versions of oboe is pulled.

# Solution

It would be great if oboe itself maintains Android.mk along side CMakeLists.txt. This will ease the adoption.

FYI. There is CMake's [EXPORT_ANDROID_MK](https://cmake.org/cmake/help/latest/command/export.html). But the mk file generated was missing lot of information.