Fixed Android 4.4+ external storage behavior

Closes #2426 

What this PR does:
- Pre-KitKat devices keep their exact behavior
- Android 4.4+ devices first check if the legacy behavior still works (All Nexus devices, some Samsung devices still use the old behavior and this PR doesn't change that)
- If the legacy behavior works, it is used; if the legacy behavior doesn't work the whole sd-card is accessible, but writing is done using the new APIs
- Hides these changes from the developer, it is all done under-the-hood.

Tested with FilesTest in gdx-tests
- Nexus 7 (2013) running Android L Developer Preview [tested with legacy + forced new behavior]
- Blu Advance 4.5 4G running Android 4.2.2 [tested with legacy + forced new behavior]

This PR does compile, I check that. :)