android: Set up Mono buildsystem

[akien-mga]

Adds support for building Mono templates for Android in 3.2/master.
Expects mono 6.0+ (otherwise we'd have to handle the tarball compression
change from upstream).

We use the script from https://github.com/godotengine/godot-mono-builds
to build Mono for all arches with the right options, and a patch included
in https://github.com/godotengine/godot/blob/master/modules/mono/build_scripts/patches/fix-mono-android-tkill.diff
which is also necessary.

I also moved the SDK to a sdk folder to make the $HOME more tidy,
especially since the Mono build scripts install the mono toolchains in
$HOME too by default.

[akien-mga]

I did the requested changes.

I also added some changes I had locally but not pushed to git yet, namely changing from using the upstream tarball to using a git clone for a specific release tag.
Mono upstream seems to have bugs in their tarball generation, and the 5.18.1.3 tarball lacked two files needed to build on Android, even though they are included in the 5.18.1.3 tag:

/root/mono-5.18.1.3/mono/metadata/../../support/libm/complex.c:19:10: fatal error: 'math_private.h' file not found
#include "math_private.h"
         ^~~~~~~~~~~~~~~~

So far this was only done for Android builds, but we could also use the same approach for Dockerfile.mono if you agree.

[akien-mga]

For the reference, while we use --branch here, we check out a git tag, so unless upstream deletes it and retags another commit, this should allow reproducible builds.
The git submodule update --init below should also update to the commit hashes referenced in the tagged mono commit, so they shouldn't change.