Fix illegal thread state bug after resuming activity

 * Used to throw IllegalThreadStateException.
 * Now thread state is changed properly when pressing
   back button and home button in various cases so
   that on resume, thread can start running smoothly
Add highscore save feature

 * Highscore table is stored in a file
 * Loaded as an ArrayList on each start-up
   and file is updated on every stop.
Add highscore display activity