set empty Gdx.* fields and made get[Component]() methods work

HeadlessApplication#get[Input/Graphics/Audio]() returned null even though HeadlessApplication had mock implementations. Correct me if I'm wrong, but I guess this was because the getters were forgotten when those mock implementations were added. I don't think they returned null intentionally because the Gdx.[audio/graphics] fields were set (Gdx.input not though, so it seems it was forgotten as well).
Merge pull request #2316 from dermetfan/master

set empty Gdx.* fields and made get[Component]() methods work in headless backend