cmake: install Android.mk files for ndk-build

This allows including SDL2 with ndk-build using
NDK_MODULE_PATH + import-module

An install tree looks like:
```
`-- share
    |-- aclocal
    |   `-- sdl2.m4
    |-- licenses
    |   `-- SDL2
    |       `-- LICENSE.txt
    `-- ndk-modules
        |-- SDL2
        |   `-- Android.mk
        |-- SDL2-static
        |   `-- Android.mk
        |-- SDL2main
        |   `-- Android.mk
        `-- SDL2test
            `-- Android.mk
```

The generated `/share/ndk-modules/SDL2/Android.mk` contains:
```
LOCAL_PATH := $(call my-dir)
_IMPORT_PREFIX := $(LOCAL_PATH)/../../..

include $(CLEAR_VARS)
LOCAL_MODULE := SDL2
LOCAL_SRC_FILES := $(_IMPORT_PREFIX)/lib/libSDL2.so
LOCAL_CPP_FEATURES := rtti exceptions
LOCAL_EXPORT_C_INCLUDES := ${_IMPORT_PREFIX}/include\
${_IMPORT_PREFIX}/include/SDL2
include $(PREBUILT_SHARED_LIBRARY)`
```
1 file changed
tree: 1c5134c9638659e480ee1e638f48fcb472a5fadf
  1. .github/
  2. acinclude/
  3. android-project/
  4. android-project-ant/
  5. build-scripts/
  6. cmake/
  7. docs/
  8. include/
  9. mingw/
  10. src/
  11. test/
  12. VisualC/
  13. VisualC-WinRT/
  14. visualtest/
  15. wayland-protocols/
  16. Xcode/
  17. Xcode-iOS/
  18. .clang-format
  19. .editorconfig
  20. .gitignore
  21. .wikiheaders-options
  22. Android.mk
  23. autogen.sh
  24. BUGS.txt
  25. cmake_uninstall.cmake.in
  26. CMakeLists.txt
  27. configure
  28. configure.ac
  29. CREDITS.txt
  30. INSTALL.txt
  31. LICENSE.txt
  32. Makefile.in
  33. Makefile.minimal
  34. Makefile.os2
  35. Makefile.pandora
  36. Makefile.w32
  37. README-SDL.txt
  38. README.md
  39. sdl2-config-version.cmake.in
  40. sdl2-config.cmake.in
  41. sdl2-config.in
  42. sdl2.m4
  43. sdl2.pc.in
  44. SDL2.spec.in
  45. SDL2Config.cmake.in
  46. TODO.txt
  47. WhatsNew.txt
README.md

Simple DirectMedia Layer (SDL) Version 2.0

https://www.libsdl.org/

Simple DirectMedia Layer is a cross-platform development library designed to provide low level access to audio, keyboard, mouse, joystick, and graphics hardware via OpenGL and Direct3D. It is used by video playback software, emulators, and popular games including Valve's award winning catalog and many Humble Bundle games.

More extensive documentation is available in the docs directory, starting with README.md

Enjoy!

Sam Lantinga (slouken@libsdl.org)