[Android] Decouple a large amount of preference handling functionality from the MainMenuActivity in the Android front-end. Along with numerous simplifications/optimizations.

- All preference handling functions are moved into their own class called UserPreferences.
- All DirectoryActivity derived classes are moved into their own package called 'org.retroarch.browser.diractivities'
- All preference related classes are moved into their own package called 'org.retroarch.browser.preferences'.
- Numerous simplifications were made, most of which are basically doing the same thing in less lines of code.

I'd recommend you test whether everything still works ok. Everything works fine for me (that I tested), but since other devs likely have better knowledge than me about internal workings, it's possible I may have overlooked some things. Either way, I believe everything works as it should.