Fixed Android 4.4+ external storage behavior

Closes #2426 

What this PR does:
- Switches reading and writing by default to the new external storage APIs which creates an application-specific directory on the sd card.
- Falls back to legacy root of sdcard if file exists there.
- 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. :)
You probably wouldn't want to create the fallback File until needed.