Implemented saving of Level Scores and displaying in the LevelSelectScreen

Scores for each individual level are now saved through the Preferences
class using a key-value map which internally saves data in XML format for
Android devices. The key format used is the difficulty name + the level.

In the Level Select Screens, the Player's score is shown by displaying
icons of the target on each Level Button. How many icons are shown is
based from how many targets the Player has hit for that particular level.