So this makes two basic changes:
1. Add a few extra dirs, hide root dir in the filesystem
2. Simplify the default dir selection code so it doesn't take three code blocks to change a dir
A few of the decisions here were weird.
This shouldn't affect anything for end users and makes the code more mantainable.
Log from the first run:
```
05-27 22:42:21.507 16529 16567 I RetroArch: jni_thread_destruct()
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV] Android version (major : 7, minor : 1, rel : 2)
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV] Checking arguments passed from intent ...
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: config file: [/storage/emulated/0/Android/data/com.retroarch.aarch64/files/retroarch.cfg] -->correct
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: current IME: [com.koushikdutta.vysor/.VysorIME]
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: libretro path: [/data/user/0/com.retroarch.aarch64/cores/]
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: android internal storage location: [/storage/emulated/0]
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: android picture folder location [/storage/emulated/0/Pictures] -->cleanup no longer needed
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: android download folder location [/storage/emulated/0/Download] -->cleanup no longer needed
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: APK location [/data/app/com.retroarch.aarch64-2/base.apk]
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: android external files location [/storage/emulated/0/Android/data/com.retroarch.aarch64/files]
05-27 22:45:27.586 16992 17046 I RetroArch: [ENV]: app dir: [/data/user/0/com.retroarch.aarch64]
05-27 22:45:27.586 16992 17046 I RetroArch: Testing permissions for /storage/emulated/0 -->correct
05-27 22:45:27.587 16992 17046 I RetroArch: Create /storage/emulated/0/.retroarch in /storage/emulated/0 true
05-27 22:45:27.587 16992 17046 I RetroArch: [ENV]: application location: [/data/user/0/com.retroarch.aarch64]
05-27 22:45:27.587 16992 17046 I RetroArch: [ENV]: default download folder: [] -->fixup
05-27 22:45:27.587 16992 17046 I RetroArch: [ENV]: default savefile folder: [/storage/emulated/0/RetroArch/saves] -->correct
05-27 22:45:27.587 16992 17046 I RetroArch: [ENV]: default savestate folder: [/storage/emulated/0/RetroArch/states] -->correct
05-27 22:45:27.587 16992 17046 I RetroArch: [ENV]: default system folder: [/storage/emulated/0/RetroArch/system] -->correct
05-27 22:45:27.587 16992 17046 I RetroArch: [ENV]: default screenshot folder: [/storage/emulated/0/RetroArch/screenshots] -->correct
05-27 22:45:27.780 16992 17046 I RetroArch: [recording] youtube streaming key empty
05-27 22:45:27.781 16992 17046 W RetroArch: Using old --libretro behavior. Setting libretro_directory to "/data/user/0/com.retroarch.aarch64/cores/" instead.
05-27 22:45:27.781 16992 17046 I RetroArch: RetroArch 1.7.7 (Git 772d06f)
```
Also these are changed:
Should be easier to navigate to other mountpoints
