Feature requeset: Android support

[wengxt]

Engine may get android support for free via https://github.com/fcitx5-android/fcitx5-android @subins2000

Though the build process is trickier to handle.

For now, it might be easier, if the build system is based on CMake, due to NDK's official cmake support. I'm not sure if it's easy to keep using meson.

Some reference resource:
https://github.com/fcitx5-android/prebuilder/ is used to build any library and data it needs.
https://github.com/fcitx5-android/fcitx5-android/tree/master/plugin for holding the extra build files used to build the plugin.