Preferences update

I extracted as many preference strings as I could find into their own string_prefs.xml files (for each language) in order to make the files more manageable and group like strings together. Additionally, I changed the names of all the preference strings to prefix them with the tag pref_ so that they will be easier to identify in code as what they are. I think, though, that this will require users to reset their preferences because the key names are changed. This would be a one-time reset.

I moved the calls to update all preferences and tiles into MainActivity.java using a sharedPreferencesChangedListener.

I modified the Preferences.java class to remove the preference updating code in the constructor, since it's now in MainActivity.java.

I updated the project target API19 instead of API17 in order to stay current with the newest Android versions.

This code tests properly on a physical Nexus 4 and Nexus 7 (both running API19), and an emulated Nexus One (running API8).