Add Android build scripts

[Jupeyy]

This is mostly finished, adds the scripts and the minimal code changes needed.
It does NOT add any functionalty.

What is missing:

• Can't get opusfile to compile for 32bit (arm and x86), probably because I set the paths wrong (I just build it manually for now and define the 64bit ftello fseeko to the c standard ones)
• libcurl probably requires openssl for https, or I forgot to enable it, LAN servers should work tho (certificate is downloaded at build time, bcs the android certs cannot be used with newer OpenSSL versions apparently)
• I didn't shell check the scripts, would be really pleased if someone wants todo it (Thanks to @ChillerDragon)
  - [ ] README doesn't mention it (could also be done after the pr is merged) (dont want todo it)
• Choose landscape mode by default (can be done in the java code)
• App icons are missing
• fix Rare(?) random(?) crash on Android #4086

fixes #4086

fixes #477

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test if it works standalone, system.c especially
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or valgrind's memcheck (optional)

[Jupeyy]

I think this is done if somebody wants to try it out

scripts/android/README.md

should explain most stuff, the easiest is to just download android studio and download NDK and SDK tools
and just run the script for your arch for testing
you can then just plugin your phone over USB and install with

adb install build_android/DDNet.apk

[def-]

Looks great, unfortunately I don't have an Android device anymore to try it out. I'd merge it to prevent conflicts in this change in the future, it should only really affect the Android build anyway. Could you resolve the conflicts?

[def-]

bors r+

[bors]

Build succeeded:

• build-cmake (macOS-latest)
• build-cmake (ubuntu-18.04)
• build-cmake (ubuntu-latest)
• build-cmake (windows-latest)
• check-clang-tidy
• check-style