Implemented SharedPreferences using User class in Main(Login) and User Registration activities

Main activity now allows checking user credentials to log in, (prompts
toast if username/password not matched).
User Registration activity now allows adding a new user into the user
list, (prompts toast if username already exists).

**Known bug: After a user is being created, I tried to go back to the
log in screen to log in using the credentials, the app now would crash.
(There is something wrong with splitting (delimiting) the user String
into an array String[ ]).