[Android] Amazon GameCenter

Use Amazon's GameCircle as native backend for `avalon::GameCenter` if `AVALON_PLATFORM_FLAVOR` is set to `amazon`.

It's important to note that `.` gets replaced with `_` in all leaderboard or achievement IDs. Why? Because Amazon has banned the dot from list of valid characters. Duh! But we should, IMHO, support to dot as it's not uncommon to put all IDs beneath the app SKU / reversed domain. 

    gc.postScore("com.example.leaderboard.foo", 42); // this has the same effect as ..
    gc.postScore("com_example_leaderboard_foo", 42); // .. this, because "." is replaced with "_"

And there is no other side effect so this can be used safely. The advantage is that IDs can be shared with the iOS backend. The difference is only visible in the developer backend and not in the code. Yay!