ci: switched to using rust-android-gradle for building aw-server-rust

[ErikBjare]

Should make it a lot easier to build aw-server-rust, which in turn will help with #24.

TODO

• [x] Remove the unused Makefile targets
• [ ] Do we need RUSTFLAGS="-C debuginfo=2 -Awarnings"? Perhaps rust-android-gradle sets it for us?
• [x] Fix openssl-sys issue
  • Seems to have resolved itself?
• [ ] Make sure it's debuggable
• [ ] Set reasonable build profile depending on build purpose (https://github.com/mozilla/rust-android-gradle/issues/38)

[ErikBjare]

lol, it passed CI? What the hell...